Hot water out of cold taps - plumber is stumped
Hi everyone! My husband and I bought a home about a month ago, and we immediately noticed that there was hot water coming out of the cold taps in the master bathroom. We had the plumber come out, and he is flummoxed. He simply can't find the cause. He's checked the cartridges for each of our single handle faucets and showers, but none of those is the culprit. He crawled under the house and determined that there isn't a plumbing issue - the cold water is cold under the house and the piping doesn't seem to be the issue. He is now proposing to cut holes in our walls to determine if any of the showers has a failed mixing valve which could be causing this.
My question is: is there anything else that we should be thinking about before my walls turn into swiss cheese? Our home inspector did note a couple of things, but I'm not sure that they're relevant (my plumber doesn't think so, but other opinions would be welcome):
* there is corrosion on the check valve for the recirculation pump for my hot water heaters (which the inspector said indicated leakage)
* there are separate temperature/pressure valves for each water heater (I have 2 water heaters in series), but the drain pipes are connected together
* there is no expansion tank (the report notes that certain recirculation systems require an auxiliary expansion
tank to relieve excess pressures from the tank)
Could any of these be the problem? What else should I be thinking about?