why is catalog timestamp updated when pc turned on?

Hi - a Photoshop Elements person suggested I post this here as well: we've put all our PE files on Dropbox, so different people in different locations can access them (at different times!), but we get conflicted copies in Dropbox, and I'm guessing this is connected to the fact that: whenever one of the pc's running PE is switched on, the timestamp for the catalog.pse12db file is updated... I've checked the Windows startup menu and startup programs and services in msconfig and there doesn't seem to be anything that would explain this. Anyone know why the catalog timestamp should be updated to the current time when a pc is switched on and PE is not running at all? Many thanks in advance.

[Moved to the Photoshop Elements forum - Mod.]