A mixing valve Tub/shower or single handle faucet can sometimes bypass through the faucet with out having to open the faucet. Old style Moen valves have a history of that occurring.
It is more apt to occurred when there is an imbalance of pressure between the hot and cold system.
One way we check to see which valve might be causing it:
If the units share a common water heater
Turn off the cold supply to water heater. If it has a recirculating pump, just reenergize it. no need to isolate the return line .
Must have a good valve that shuts off 100%. best results if you have a 2 handle faucet somewhere in one of the units.
Open the hot side of one faucet. should drain down and stop flowing 100%
If you get any continuous flow, then cold water is crossing through somewhere.
Now go listen to every faucet you have access to and you can probably here the water hissing through the culprit that is causing your problem.
Sometimes you don't have access to all the units. But the more you can check at one time the better results.