Sudden leak from toilet after years of no problems
Yes, wax rings can "dry out" when it breaks down causing shrinkage, hardening, and cracking.  In your case, an old(er), rarely used toilet with a dried out wax ring might have broken when someone sat on the toilet.

Leakage would also be noted around the base of the toilet though, if it's not a lot of water, may be hidden by the toilet base.  If one puts his nose to the toilet base and smells sewer gas...yep...broken ring.

Note: One should periodically run water in all bathroom fixtures to ensure water in P-traps doesn't evaporate.

Removing a toilet is not difficult.  Buy a new wax ring and replace the one you have.  If water stains continue, move on to some other possibility. 

No.  There's no characteristic of PVC drain pipe that would lead to leaking after extended disuse.  But you state water is run periodically; therefore, there's no real disuse...just not frequent use.