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Water collecting beneath Tile

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We made our bathroom larger and I had to remove several partial tiles in order to continue to tile the new section. The existing floor has 18 X 18 porcelin tile on a concrete slab with polyurethane grout. When I removed the tiles there was water beneath them, collected in the gaps in the mortar left by the trowel. My first thought was the toilet gasket was leaking but after removing the toilet there was no evidence of leakage. Next I tested for a water leak in the slab by reading the water meter and waiting 4 hours. The meter showed no flow. None of the sink supply or drains show any evidence or leakage. The bathroom has a shower stall but there is a 3" step down into it.
How likely is it that small amounts of water from stepping out of the shower would penetrate the grout and collect beneath the tiles? Even if this did happen I would think the concrete slab would absorb and dissipate the water.
Before I tear up all the tile, any thoughts on a source of this water?

Thanks
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