As other posters have indicated, any faucet or appliance that's connected both to hot and cold (that generally rules out
outdoor connetions) could be the culprit.

Or... there's something called a "tempering valve" that mixes hot + cold water to reduce the temperature. Often used to
extend the hot water supply by allowing the heater to be set higher, then adding cold to make the desired temperature.
They're usually located near the water heater, but could also be near a fixture.

Eric Law