Water pipe puzzle

[Woodywhite]

Anyone like doing puzzles? I've got a 1960, 18' Traveler (my first trailer) I bought about two months ago. I've worked through the rest of the systems and started to check out the fresh water system. As I tracked the system from end to end, I have found what appears to be an open pipe out the belly with no apparent way to shut it off.

Pipe comes from the fresh water tank, which goes to T1.
T1 to the right is drain, with the red handle, which drains out the belly
T1 to the left goes to T2.
T2 to the right goes to the sink and hot water heater
T2 to the left goes to T3
T3 straight goes to the city supply line below the belly.
T3 left goes to the mystery line, which seems that it would continuously drain the system.

Plumbing should be pretty straight forward, but this makes no sense to me. Am I missing something?

Thanks,
Bob