The most likely place where the water is leaking is from one of the PEX joints. Another possibility would be one of the compression fittings where the PEX hooks up to something like a faucet. In my rig, the compression fitting for the water hookup for my toilet has started leaking several times. Each time I have had to tighten it just a little bit more to get it to stop. I finally got it to stop after several iterations of hand tightening.