I have a strange leak that has me baffled.

There are 2 houses on my property  both fed from a centrally located well but each house has its own septic system.

The leak occurs in a bathroom, I think its leaking from the drain pipe but I'm not positive.

The strange thing to me is that it only leaks when the water is shut off outside at the well pump and the pressure is bled out of the system.

It isn't a huge deal because I don't shut off the water very often but I wish I knew what was going on. 

Any one-way valves that may only be maintaining a seal under pressure?

thats the best theory so far.

The house with the leak is downhill from the other house so maybe the water flowing downhill is not enough pressure to seal the valve.

Also found out that the leak occurs when a lot of things are turned on at the uphill house at the same time, like water hose and dishwasher, and washing machine.

when a lot of water is being used at the other house the pressure here is barely a trickle.

Bummer. Any way to get to the leak's location without ripping out drywall or a floor?

was agreeing with ryott . is there a way to feed each home seperately without major expense? have a similar situation going on now. 2 homes one  is non habitable atm. vandals etc.  but its inprobate so i cant do much lol