Water under a tile floor can be caused by a various number of things. Knowing if you do have water underneath tile is important. This can generate mold that can be hazardous to humans and animals. It can also cause damage to the subfloor or wood underneath the tile.
Leaking drainpipes in a shower or a bathroom plug will let water underneath tile. The best way to tell if this is your problem is to push down on the floor around the drain. If it gives, this may be the problem. Another way to tell is to examine the drain from underneath, which can be accomplished in the crawlspace or the basement. Dripping water would be a key to leaking drainage.
If the grout becomes damaged on the floor, it will give water a chance to slip through the gaps and under the floor. If you see damaged grout, you need to mix a small amount and replace it as soon as possible, which is especially important in areas that are exposed to water from entrance ways.
Poor Drainage From Outside
If you have a section of the house exposed to an area where rainwater or melt-off from snow is present and it is contacting masonry, you may have a leakage problem from outside, which can be remedied by applying a plastic sheet to the outside of the house or contracting to have a French drain installed. Both methods will lesson the pressure of the water on the masonry.
If there is a damaged or cracked tile, this will also give access to water. It will not require a large amount of damage to allow the water to penetrate. Something as small as a hairline crack will allow moisture to pass through. These tiles need to be replaced as soon as you notice they are damaged.