This has happened to me a few times. I have traced the problem to a drop sink in my garage which has a spray nozzle like you’d find in the dish pit of a commercial kitchen/restaurant.

There are separate valves for hot and cold that blend into the faucet and the sprayer. The faucet has a valve which can be closed to send all of the water through the sprayer. Usually, when I am not using the sink I leave that valve open and turn off the hot and cold valves individually, but every once in a while I forget to do that and i leave the hot and cold open, closing the valve to the faucet instead (leaving the line to the sprayer charged).

Whenever I do this I get warm water from the hot taps around the rest of my house. I think the cold water supply has more pressure than the hot and it’s forcing it’s way through the valves in that drop sink and lowering the temps in my water heater. This has happened a few times now and it’s the first thing I check if I’m not getting hot water in my kitchen or bath.

I hope I explained that well enough. It’s 3 AM.