Check to see if the cold water line to the water heater is allowing hot water to rise up in a thermal siphon and enter the cold water main. feel the cold water dip pipe going to the water heater to see if it is warm. If it is hot, check the dip tube on the waterr heater to make sure it has not corroded away. if it is hot consider adding a check valve on the CW supplt to the heater or put a heat trap in the piping. (an S shap pipe layng on its side with about an 18 to 24 inch change in elevation)

If that is not the problem check all sink faucets to see if there is a crossover problem where hot water can be crossing over into the cold water pipe. Some cheap faucets do not have check valves on the inlets to prevent crossover flow.

Sometimes when there is a faucet with a hose valve like in a laundry the hot can cross over into the cold. Check all fictures with both a hot and cold water connection to see if the cold water pipe is warm when you open the faucet that experiences the hot water.
Good Luck

Do you have a hot water recirculating pump installed? Let me know, Tom