I had a similar thing in my 85 300D. When my sunroof switch sticks in the 'close' position, I get a horrible squeal from that area the next time I start the car. The sunroof motor is right next to the antenna motor. I'll bet that's what you're talking about. I can see why you think it is an alarm. That sucker is LOUD.

Also, in mine, the antenna won't go up until the radio is turned on. Might be she just didn't have the radio on.