The best way to tell if.
Water leaking through tile grout.
If the grout becomes damaged on the floor it will give water a chance to slip through the gaps and under.
Leaky tiles can be found anywhere where there is water.
The grout joints are porous so water can go through the grout and under the tile.
The shower water leak should be repaired quickly otherwise the buildup of stagnant water will lead to mold and damp under the tiles.
There should be proper flashing the membrane should be sloped to a drain that has weep holes to allow the water through.
This tile needs periodic maintenance.
If water is oozing up through the grout joints and it is becoming moldy then water is not able to drain away.
Check the ceramic tile walls surrounding your tub or shower.
Maybe a plumbing leak or maybe the drain is restricted in the shower and water is not allowed to fully drain away.
If it is leaking then it isn t fully waterproof.
Leaking drainpipes in a shower or a bathroom plug will let water underneath tile.
It appears you have some sort of leak.
Cracks can develop in the grout between the tiles allowing moisture to seep through.
They may have waterproofed the underlying deck but they could have made mistakes.