This post on reddit made me go through my logs. That's when I discovered the following entries which appeared on two non-subsequent days. "user" is my user account.

Aug  4 22:50:37 UbuntuSystem sudo: pam_unix(sudo:session): session opened for user root by user(uid=1000)

Aug  4 22:50:39 UbuntuSystem sudo: pam_unix(sudo:session): session closed for user root

Aug  4 22:51:16 UbuntuSystem su[10710]: Successful su for user by root

Aug  4 22:51:16 UbuntuSystem su[10710]: + ??? root:user

Aug  4 22:51:16 UbuntuSystem su[10710]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10710]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10720]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10720]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10720]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10720]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10735]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10735]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10735]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10735]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10763]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10763]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10763]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10763]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10773]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10773]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10773]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10773]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10788]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10788]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10788]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10788]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10801]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10801]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10801]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10801]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10814]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10814]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10814]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10814]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10829]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10829]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10829]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10829]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10842]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10842]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10842]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10842]: pam_unix(su:session): session closed for user user

Aug  4 22:51:17 UbuntuSystem su[10855]: Successful su for user by root

Aug  4 22:51:17 UbuntuSystem su[10855]: + ??? root:user

Aug  4 22:51:17 UbuntuSystem su[10855]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 22:51:17 UbuntuSystem su[10855]: pam_unix(su:session): session closed for user user

Aug  4 23:41:39 UbuntuSystem su[11153]: Successful su for user by root

Aug  4 23:41:39 UbuntuSystem su[11153]: + ??? root:user

Aug  4 23:41:39 UbuntuSystem su[11153]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 23:41:39 UbuntuSystem su[11153]: pam_unix(su:session): session closed for user user

Aug  4 23:41:39 UbuntuSystem su[11166]: Successful su for user by root

Aug  4 23:41:39 UbuntuSystem su[11166]: + ??? root:user

Aug  4 23:41:39 UbuntuSystem su[11166]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 23:41:39 UbuntuSystem su[11166]: pam_unix(su:session): session closed for user user

Aug  4 23:41:39 UbuntuSystem su[11181]: Successful su for user by root

Aug  4 23:41:39 UbuntuSystem su[11181]: + ??? root:user

Aug  4 23:41:39 UbuntuSystem su[11181]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 23:41:39 UbuntuSystem su[11181]: pam_unix(su:session): session closed for user user

Aug  4 23:41:39 UbuntuSystem su[11193]: Successful su for user by root

Aug  4 23:41:39 UbuntuSystem su[11193]: + ??? root:user

Aug  4 23:41:39 UbuntuSystem su[11193]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 23:41:39 UbuntuSystem su[11193]: pam_unix(su:session): session closed for user user

Aug  4 23:41:39 UbuntuSystem su[11211]: Successful su for user by root

Aug  4 23:41:39 UbuntuSystem su[11211]: + ??? root:user

Aug  4 23:41:39 UbuntuSystem su[11211]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 23:41:39 UbuntuSystem su[11211]: pam_unix(su:session): session closed for user user

Aug  4 23:41:39 UbuntuSystem su[11226]: Successful su for user by root

Aug  4 23:41:39 UbuntuSystem su[11226]: + ??? root:user

Aug  4 23:41:39 UbuntuSystem su[11226]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 23:41:39 UbuntuSystem su[11226]: pam_unix(su:session): session closed for user user

Aug  4 23:41:39 UbuntuSystem su[11241]: Successful su for user by root

Aug  4 23:41:39 UbuntuSystem su[11241]: + ??? root:user

Aug  4 23:41:39 UbuntuSystem su[11241]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 23:41:39 UbuntuSystem su[11241]: pam_unix(su:session): session closed for user user

Aug  4 23:41:39 UbuntuSystem su[11253]: Successful su for user by root

Aug  4 23:41:39 UbuntuSystem su[11253]: + ??? root:user

Aug  4 23:41:39 UbuntuSystem su[11253]: pam_unix(su:session): session opened for user user by (uid=0)

Aug  4 23:41:39 UbuntuSystem su[11253]: pam_unix(su:session): session closed for user user

Aug  4 23:42:18 UbuntuSystem gnome-screensaver-dialog: gkr-pam: unlocked login keyring

Aug  4 23:42:33 UbuntuSystem polkitd(authority=local): Unregistered Authentication Agent for unix-session:/org/freedesktop/ConsoleKit/Session2 (system bus name :1.48, object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_US.UTF-8) (disconnected from bus)



Aug 15 20:17:01 UbuntuSystem CRON[26579]: pam_unix(cron:session): session opened for user root by (uid=0)

Aug 15 20:17:01 UbuntuSystem CRON[26579]: pam_unix(cron:session): session closed for user root

Aug 15 21:15:15 UbuntuSystem su[27098]: Successful su for user by root

Aug 15 21:15:15 UbuntuSystem su[27098]: + ??? root:user

Aug 15 21:15:15 UbuntuSystem su[27098]: pam_unix(su:session): session opened for user user by (uid=0)

Aug 15 21:15:15 UbuntuSystem su[27098]: pam_unix(su:session): session closed for user user

Aug 15 21:17:01 UbuntuSystem CRON[27141]: pam_unix(cron:session): session opened for user root by (uid=0)

Aug 15 21:17:01 UbuntuSystem CRON[27141]: pam_unix(cron:session): session closed for user root

Apart from these iterations the only other times I found a similar output was when trying out the guest account:

Aug 11 22:38:49 UbuntuSystem lightdm: pam_unix(lightdm:session): session closed for user lightdm

Aug 11 22:38:49 UbuntuSystem groupadd[2918]: group added to /etc/group: name=guest-4Eflre, GID=125

Aug 11 22:38:49 UbuntuSystem groupadd[2918]: group added to /etc/gshadow: name=guest-4Eflre

Aug 11 22:38:49 UbuntuSystem groupadd[2918]: new group: name=guest-4Eflre, GID=125

Aug 11 22:38:50 UbuntuSystem useradd[2922]: new user: name=guest-4Eflre, UID=115, GID=125, home=/, shell=/bin/bash

Aug 11 22:38:50 UbuntuSystem usermod[2927]: change user 'guest-4Eflre' password

Aug 11 22:38:50 UbuntuSystem chage[2932]: changed password expiry for guest-4Eflre

Aug 11 22:38:50 UbuntuSystem chfn[2935]: changed user 'guest-4Eflre' information

Aug 11 22:38:50 UbuntuSystem usermod[2943]: change user 'guest-4Eflre' home from '/' to '/tmp/guest-4Eflre'

Aug 11 22:38:50 UbuntuSystem su[2948]: Successful su for guest-4Eflre by root

Aug 11 22:38:50 UbuntuSystem su[2948]: + ??? root:guest-4Eflre

Aug 11 22:38:50 UbuntuSystem su[2948]: pam_unix(su:session): session opened for user guest-4Eflre by (uid=0)

Aug 11 22:38:50 UbuntuSystem su[2948]: pam_unix(su:session): session closed for user guest-4Eflre

Aug 11 22:38:50 UbuntuSystem lightdm: pam_unix(lightdm-autologin:session): session opened for user guest-4Eflre by (uid=0)

Aug 11 22:38:50 UbuntuSystem lightdm: pam_ck_connector(lightdm-autologin:session): nox11 mode, ignoring PAM_TTY :0

I might have to add that I set up my system only fairly recently (Aug 4).

Is this behaviour normal? What exactly is going on with all the su commands? Do I have to be worried that my system might be compromised?

Many thanks in advance.