Alarm/Lights

I have noticed the every so often, when I return to the X5 and unlock the doors/disarm the alrarm it will only flash the lights and beep once like it is being locked/armed.

I checked and I didn't forget to lock the car...and the alarm did not go off while I was away/asleep, so I am at a loss as to why the different indicator. I looked in the manual and didn't see anything...anyone have an idea what might be occuring?