High-tech Protech. Thermal imaging ?
Have cold water crossing over into the hot side.
I already shut off all of the stops in the house including the laundry and water heayer inlet valve. <snip>
YOU SAID LAUNDRY. I assume that means washing machine.
Did the stops work? Is it a symmons or watts? [sometime the handel says off but the valve itself never moved with the handel. I have seen major cross overs at wash machines. It crosses right through the selonoid, when the washer is not in use. Also green house is customer using hose Y connections to get tempered water for plants. No checks installed ...
A hard piped crossover that didnt cause problems for 5 years is highly unlikely.
There are no manifolds. Its trunk and branch.Can you electronically trace to the manifolds? Plastic will be difficult to locate.I would look first at the manifold behind the water heater. Disconnect all manifold lines and then I would connect a hose to the backflow and connect each individual manifold lines to a hose and see which one feeds the hot line behind the heater. Reconnect the manifold lines with exception to the cross connected line and discover what does not have water