Unearth the aircraft battery, look to see if there are any connections
to the positive pole upstream from the master relay. You might find
one: the clock keep-alive circuit which should have a 1A cartridge
fuse in series with it. If there are any other connections upstream of
the master relay, you need to trace them to see where they go.

I have a SP400 intercom in my Pacer, and it is powered through its own
circuit breaker from the main avionics bus. With the master off, it is
unpowered, and it would not matter if headsets were plugged in or not.
It supplies the mic current to the backseat headsets, so if it is off,
they are off too. Sounds like some dumb**** who didn't have a clue
installed the intercom!

A common problem with older electrically-powered self-winding aircraft
clocks is that the winding solenoid intermitently sticks on, which will
run down your battery is short order.
As the clock spring runs down, a switch closes, applying power to a
solenoid, which is supposed to "wind UP" the spring, causing the switch
contact to open again. If the switch stays on, the solenoid remains
powered... Best fix is to chuck the mechanical clock and replace it
with a LCD pure electronic version...