It MAY be that someone has caulked the bottom edge of the shower surround which traps condensation behind the panel and can cause all SORTS of problems. Removing the surround to check for damage seems like a good idea but when it (or a new one) is installed be sure to NOT seal around the joint with the tub. Perfectly fine to caulk all other sides of the surround, just not the bottom. This is often one of the FIRST things a new owner might do, thinking they are protecting their new investment.
Good luck / Skip