These kinds of “leaks” I normally take a wait and see approach.
The best advice I could give you was given by daveroconn.
Sooner or later this leak will show up THEN it will be much easier to find the cause.
Could be drain line or grouting as a water supply leak would be more consistent.
One more option is to make sure all your faucets are shut off tight and then check your water meter is applicable this also will show you if water is being used. CHECK your toilet tank for a possible over flow into the over flow tube from a faulty ball cock or defective tank bulb or flapper.