Thanks for all the input.

There are four seperate meters in the basement. To access the basement
you have to go through one of the apartments (not mine) which is why
the landlord, nor anyone else, didn't find out about the water heater
for some time.

I'm going to ask the landlord if he noticed a higher water bill this
month. He's replacing the hot water heater tomorrow. So if the water
bill is higher and the gas bill drops as a result of the new one I'll
know for sure and hopefully be getting compensated.