locked pc unlocked by itself - is it possible? :o

Hello, I have a colleague in our company who locks pc when he goes
away and he claims that sometimes he gets back and his pc is unlocked!
Do you think this is possible without someone else knowing his
password and making a fun of him? He ensures me, that nobody knows his
password because he changed it...
I checked the event viewer and there are only 3 events when the pc
gets unlocked (logon type: 7), but no event when the pc is locked, so
I cannot tell for sure it was locked...
Is the locking of pc logged somewhere or is there some registry value
changed somehow to see that the pc is locked?
.