UPDATE: here's a video of the situation: https://youtu.be/NoTIYk_x8EU In this video the hot water heater valves are closed. There is no recirculation. All sinks and toilets are closed at the valves. All the shower cartridges are removed like the ones shown.
If I disconnect the water heater and turn on the main, cold water flows through BOTH pipes that go to the water heater. The cold line (correctly) and the hot line (this is backwards).
I have turned off the value on both lines, turned off all the valves on all the sinks and appliances. The only thing left is the shower valves. I thought it might be a valve, so i removed the cartridges from all of them exposing hot and cold holes in the valves.
I turned on the cold water (the water heater is completely shut off with valves on both pipes) and of the 3 showers, only the shower on the bottom floor had water come directly out of the valve. If i put my finger in to plug the cold water hole, cold water also comes out of the other. If i remove my finger, the hole that's supposed to be hot loses pressure but still flows quite a bit.
The lines are definitely crossed but i can't see where. The house is made of concrete and cinder block (this is typical in this area of mexico) so it's not easy to break open the walls. Is there any way I can detect where this crossover is? Is there anything else it can be?