You come across a bunch of statues that are obviously the result of a medusa's or basilisk's petrification.
The PC's detect magic on the bodies of the statues or in the backpack.
They remove the backpack... chisel it off the statue. They take their time to do a good job so the backpack is not harmed.
There are no particular rules for un-petrifying objects, as (as far as I know) nothing in the rules say that objects can or cannot become petrified. The simplest solution, however, would be just to say that they cast Remove Affliction on the entire statue, reverting the creature and all its gear (assuming you subscribe to the theory that it was turned to stone as well) back to its original state.
Personally, if they just wanted to un-petrify the backpack I'd just handwave it to work simply to avoid the above debate on the whole issue of whether or not powers and other things work on objects and whether objects can become petrified at all.