It sounds like one or more of the single handle valves in your building has degraded and has an internal cross-over. The fix is either replace the valve or install a new cartridge. Some brands are known for this. With them, it's not a matter of if, but of when they exhibit this behavior. It could be in your unit or another, depends on how the hot water is routed. If you have your own WH, then it's in your unit.
What brand(s) of single handle faucets do you have? Also, if you have an apartment style washing machine that you connect to a sink faucet, that might also be a source of the cross-over.