The outlet in Bathroom 1 has a GFI. There is one line feeding the line side, and
there are two lines on the load side. This same GFI is used for the outlet in
Bathroom 2 since the BR2 outlet loses power when the GFI on BR1 trips.

The GFI is tripping even with nothing connected to either bathroom outlet. The
problem is that there is obviously something else connected to the circuit, but
I cannot figure out what it is. As I said, there are two lines on the load side
of the GFI. One of them feeds the outlet in Bathroom 2. I have no idea where the
other one goes, so I can't find out what on that circuit is causing the trip.

Even further, there are two lines tied together on the Bathroom 2 outlet, as if
something else is fed off of that outlet. How can I find where the other wires
go? They're in the wall. The GFI is not bad. It was just replaced. It doesn't
trip if I power the GFI but leave the lines on the load disconnected.

What else is likely tied to the GFI?