I read through this topic the other day and didn't post because I couldn't see that I could add any help. :shrug:
I'm still not sure I can help but to clarify some things, all my fets bar one use mica shims to prevent contact of the metal fet back plate on the block. This was not really a requirement as all the fets tested "earth" on the back plate and as my water block is also earthed it wouldn't matter. I just did it for additional safety / piece of mind. My water block is earthed via a wire to the PSU chassis, so in theory I could have left the mica sims out as they were originally stopping the live 240volt mains sink from shorting out on the earthed fet back plates. Those that worry about the mica shim not isolating the voltages should realise this is what they do and are designed to do in the original item. Also look upon the mica shim in a similar way to the plastic coating around the wires, no one questions this thin plastic coping with the voltage isolation.
The fet that has no mica shim has a plastic backing with no exposed metal. On my fets in my PSU, (and you have to appreciate all PSU's differ in design, components and layout), the washers I used were purely to spread the loading on the fet package and not for isolation. The metal area around the fet hole on the front is not connected internally or externally to the metal backing on my fets.
I'm puzzled as to what is causing your sinks to show a voltage, and can only assume it is as
Prlwytkovsky said, capacitance, and the voltage wont have any amperage, (or a very small amount), I'm only guessing though as I don't really understand enough about how they work to advise
A long shot the sink is not touching the tops of the large coils??