I would imagine if you kept notice of what number series was left on the locked safe, it would leak a little bit of info. for instance, if the last number of the combo is 42, but when its locked you consistently see the number 10, its likely the person spins the dial a predictable number of times each time he locks it. all you'd need is to observe him -from a distance- locking it and count the number of spins and you'd have a good guess what the last number is.

then there's the paddle technique, not sure what its called, but after the first number is set, the 2nd number has to be at least past the point where you feel the "paddle" contact the 2nd wheel in the series. Higher-end safes counter this somehow.