Beeping noise when entering truck?

[JThiessen]

Last night after my eye Dr appointment, I unlocked the doors on my 06 with my key for, and as I sat down, I noticed a beeping noise. This started prior to me putting the key in the ignition, I quickly looked around to see if anything was flashing at me, and didnt see anything. I inserted the key, turned it on, and it stopped. I then looked for the first time at the security light, but it didnt keep blinking. I got out and quickly scanned both sides of my truck, and didnt find anything out of the ordinary. Its a loaded Laramie, doesnt have the alarm, but it has the sentry system. Anyone have any guesses as to what this may have been? I guess there may be a chance it could have been my XM with a lost signal, but I swear the noise started prior to inserting the key.