dude... i got the same problem, till i realised youtube allows multiple log ins....so if you have signed in with the "keep me signed in" on another comp and forgot to sign out... the other party can access your account at anytime even if he closes that window..
go to your account info on youtube.. there is an option there to sign out on all accounts...what that does( if it does anything) is it signs you off from all the computers you have signed in on...i realised that when i kept having transformers cartoons on my recent videos...mate's kid loves transformers...
also... this may not work all the time, i reckon the other party must have youtube open for that to work.. so whenever you see discrepancies, do the logg out on all computers... i kept doing that for a week, and its stopped somehow...