If the grout is intact and the water isn’t pooling on the tile anywhere it’s probably something to do with the drain/plumbing. If you’re tearing it out it shouldn’t be too hard to determine where the problem was. If you can’t find anything obvious I’d probably call someone with professional knowledge to take a look and see if they can identify the issue. It’d be more likely to be a spot where the floor meets the wall than just soaking through the grout unless it’s cracked.