Logout off - yet still closes user

I have a user where the Preferences/Logout tick box is blank - yet - the client still logouts at random, and quite frequent intervals.This is a Mac client so checking the ini file isn’t an option.

Any ideas before the put a foot through a screen??

Subject: Logout off - yet still closes user

The F5 button will logout - are they hitting that (either acidentally, or presuming it does something else) ?