:Cross feed in a faucet is unlikely. Due to a pressure drop across an appliance like a water heater, the hot line would generally be at a very slightly lower pressure than the cold.

: Siphoning is unlikely, but if the water heater dip tube is broken, maybe..

: You did not mention elevations; how many story house? Is the water heater higher that the piping system in the house?

: By any chance , do you have a retrofit recirculating system to save water, ie the Laing or Metlund systems? If so , this is the answer, because a normal side effect of such systems is to have 80degree water in the cold pipe .