Whohas (if I recall correctly) doesn't work quite right if someone opens a drawing via explorer, that is, they double click it whilst in Windows Explorer.
A work around that I did (again, if I recall correctly) was to pen a little uility that fires on start up -> if WhoHas isn't loaded it writes out the same data (loginname, date, time).
Something like that. I'll try to remember to look this up later today.