About 15 years ago, the Father in Law had exactly the same problem. Their house was on a slope and he had found about a foot of gin clear water under their floorboards when he had them up to do some wiring. As far as I can remember, he tried all the same tests as the OP but never got to the root of the problem. In the end he knocked a hole through his footings and fitted a drain pipe to let it out the rear of the house and into the garden.