I installed wireshark recently. I have no idea in which directory it was installed, but it's not in /opt.
How do I find where it's installed?
Assuming you installed it from the repos:
$ dpkg -L wireshark wireshark-common
/.
/usr
/usr/bin
/usr/bin/wireshark
/usr/share
/usr/share/menu
/usr/share/menu/wireshark
/usr/share/pixmaps
/usr/share/pixmaps/wsicon32.xpm
/usr/share/applications
/usr/share/applications/wireshark.desktop
/usr/share/doc
/usr/share/doc/wireshark
/usr/share/doc/wireshark/copyright
/usr/share/icons
/usr/share/icons/hicolor
/usr/share/icons/hicolor/48x48
/usr/share/icons/hicolor/48x48/apps
/usr/share/icons/hicolor/48x48/apps/wireshark.png
/usr/share/icons/hicolor/scalable
/usr/share/icons/hicolor/scalable/apps
/usr/share/icons/hicolor/scalable/apps/wireshark.svg
/usr/share/man
/usr/share/man/man1
/usr/share/man/man1/wireshark.1.gz
/usr/share/doc/wireshark/changelog.Debian.gz
/usr/share/doc/wireshark/README.Debian
/.
/usr
/usr/bin
/usr/bin/editcap
/usr/bin/text2pcap
/usr/bin/dumpcap
/usr/bin/rawshark
/usr/bin/mergecap
/usr/bin/capinfos
/usr/share
/usr/share/doc
/usr/share/doc/wireshark-common
/usr/share/doc/wireshark-common/README.Debian
/usr/share/doc/wireshark-common/copyright
/usr/share/lintian
/usr/share/lintian/overrides
/usr/share/lintian/overrides/wireshark-common
/usr/share/man
/usr/share/man/man4
/usr/share/man/man4/wireshark-filter.4.gz
/usr/share/man/man1
/usr/share/man/man1/mergecap.1.gz
/usr/share/man/man1/capinfos.1.gz
/usr/share/man/man1/dumpcap.1.gz
/usr/share/man/man1/editcap.1.gz
/usr/share/man/man1/text2pcap.1.gz
/usr/share/man/man1/rawshark.1.gz
/usr/share/doc/wireshark-common/changelog.Debian.gz
