One thing I didn't see mentioned here: some companies, especially in finance, have mandatory vacation for another reason. Forcing people to take time off prevents them from becoming critical and make sure that the rest of the team is able to handle their absence.

There's a more cynical reason, too. If you're up to no good, the rest of your team is more likely to notice when you're on vacation, they're covering for you, and whatever you came up with to hide your malfeasance is on hold.