I know this b/c of the door pressure test that was done prior to passing inspection for the Inergen fire system.
I am not sure I know what you mean by this are you talking about a blower door test?
You really dont want a another system in play in the room especially only a return and here's why when your room reaches design temperature (the temp you set your thermostat too) and the main building a/c is on what will happen is that return will suck your conditioned air out of the room and since no air is being put back in the room by that system it creates a negative air pressure in the room which means the room is going to be sucking air in from wherever it can cracks in the walls,doors, exterior windows, ext anywhere it can to stabilize the pressure void caused by the air being sucked out by that return. So not only does it suck out your cool air it draws in hotter air to the room this will cause your unit to cycle more cause every time it reaches design temp when the main a/c is running the air gets immediately sucked out.
This is not good. Make sure it really is sealed personally I would insist on that return being capped off if I was installing your system but if what you are telling me about
the test you had done if it was a blower door test it would tell you how much infiltration and would be extremely accurate if it was done correctly but I am still not sure I would take the chance as once the new system is installed the return has no reason to be there other than to cause a problem. To begin with that design was not good as air should be balanced. It is impossible to be perfect but the goal is to put back in the same amount of air that you take out of a room so neither a positive or negative pressure condition occurs here that wasn't even being attempted.
What size is this return?