Hot water problem

[PSK480]

This may be seen as a d|ck move but if you're at your last wit I would suggest you can trace the lines off the hot water heater and when you find their's turn it off or turn it down. Not many people would try this, but, you'll know pretty quick if the reason you're thinking of is the real reason. That or you should be able to hear the water running past you in the pipes if their apartment is above yours. Especially since they probably ran the bathrooms one right above the other.