I'm trying to figure out why my hallway was wired in such as strange fashion:
I have two hallway lights and three switches. Switches 1 and 2 appear to be 3-way switches; if they are both ON or OFF, light 1 is on.
Switch 3 appears to control Light 2 if and only if Light 1 is ON, otherwise it has no effect.
The wires in the junction box for Switch 3 are illustrated below.
When Light 1 is on: When Switch 3 is OFF, Terminal 2 = 120V, T1=0v. When Switch 3 is ON, T2= 120V, T2=120v
Things get confusing when light 1 is off: When Switch 3 is off, the voltage at Terminal 2 is about 12 (not 120), and when Switch 3 is on, it is about 2.
My guess is that the bottom of Switch 3 leads to Light 2, and that the top left of the box for Switch 3 is the return, while top right goes to Light 2.
Is this plausible? Correct? Common?