It is very likely that the base is leaking. If it was a precast unit, then it had a metal tile flange around the perimeter and that flange could have rusted out after 22 years and then the water will leak into the wall. If it is a cast in place unit then the safety pan may have deteriorated. From the way the question is phrased, I assume it is the first type, because they were installed without a safety liner.: Is it possible for a 22 year old cast shower base set on a concrete floor to crack or develop leaks? Or is it more likely that the drain is leaking and getting under the base?
