so, a reflex pir on average house alarm circuit will read around 20Ω measured at the controls.
whats on these problem system
eg - 3 keypads, 2 external sounder sounders, 7 pirs, 4 door contact a PA button & sound bomb ?
you sealed the hole in the pir case & where the cable enters cable come though the hole ?
you measure induced ac ?
these reflexes on brackets or screwed to the wall, how many screws are you using....
whats day / alarm current ?