I think you have ground water problem, probably from roof run-off and
rain that is a bit above the basement floor. I'm assuming that the
washing machine drains to a sanitary sewer, not just under the slab.
The PVC pipe you describe is likely not the culprit. If you are still
concerned just drop a sewer dye tablet into the washing machine drain
when the machine isn't running.. If the water coming up through the
floor shows color, then the drain is defective, if not it isn't the
source of the water.

I think we have a winner.