Why does my Audi beep and flash after I lock it?

I have an 1998 Audi A4 and every once in a while, after I lock it the lights flash and it beeps once a minute or so. All the doors were closed, and I locked it with the key so I don't see why it does this. Any answers?|||Were any of your windows partially open? The car has interior motion sensors that are very sensitive. Even a gust of wind can set off the alarm.|||It is its way of saying au revoir to you until next time.