Years ago (we're talking 15+ years now) I used to work as an alarm/stereo installer. Given that it was so long ago, this may no longer be relevant. I haven't installed an alarm in ages.
At that time, I installed Viper alarms (and other DEI brands). There were two ways to hook one up: 1) Voltage sensing or 2) switch sensing.
Voltage sensing was an easy install, but less reliable and less effective. Voltage sensing would basically detect a minute change in voltage and determine that a door or trunk had been opened, thus turning on the dome light (or trunk light). It was a much simpler install because you didn't have to hunt for all the correct wires for triggers or install pin switches on the hood and trunk. We would charge less for a voltage sensing install.
Switch sensing was a more complicated install, but more reliable. You didn't have to worry about all the "what if's". i.e. What if the car has a slow fade dome light? - what if the bulb is burnt out? - what if the dome light is switched off? - what if the battery starts to go bad? (voltage sensing alarms go crazy when that happens).
If things haven't changed since I used to install (again, it was quite a few years ago), it sounds like your installer did yours as voltage sensing.
