There was a great response to this question of the "3 Beeps" on this forum, but I can't find it. It basically said that: on '06 models and older, as I recall, that the detection of the fob was defective, but there's a technical update (applied via Tech2) that fixes it. It seems that our fobs are determined to still be inside, even when we walk past the left front of the car before the car goes into the locking mode.
My fix? Just do not walk close to the left front fender after closing the door, for about 8 seconds.

I find it a good opportunity to check out the left rear of the car, etc.
Someone with better search capabilities than I could post the link to that thread [hint...].
FWIW
Tim