clifford g5 arrow chirp issue

I have a 2001 Nissant Frontier SE Crew Cab.  I installed the Clifford G5 Arrow system about 4 years ago or so.  No issues until now.  It's really not that big of an issues, I would just like to know what's causing it.  About every other week or so, when I turn off the engine and pull the key out, the alarm will chirp once. This has been happening for about a month or two now.  I have the auto lock feature on.  So that when I start the truck it automatically locks the doors.  Like I said, it's not often enough for me to know exactly what I'm doing when it chirps.  But, it's usually when I'm grabbing all my stuff to get out of the truck and walk into work.  Any ideas?  Thanks!

Could it have been set for passive immobilisation?

No you would have to do that, having said that I've been installing Cliffords for nearly 20 years and I've pressed the wrong button combination often enough even on my own cars.

ISTR when I had an Arrow 5 installed it would chirp if the ignition was turned off, then on and off again within a short time. Could it be a sign of an ignition switch on the way out?

No, it's simply noting the ignition recycling.