People think that the tile and caulk in a shower are the waterproofing layer, but the real waterproofing is beneath the surface. A properly built shower should be waterproof even before the tile are installed. The leaks could be from the supply lines going to the valve, or the elbow or shower arm connection, or, it could be a bad shower pan, or a compromised curb, or other things. First thing I'd do is take the trim off the shower valve and see if it looks wet there, then if dry, you might then need to start looking elsewhere. Are there any areas outside of the shower in the bathroom that look wet?