Strange alarm fault

I have a strange alarm fault developed on my audi a4 (2001).
Once the outside air temperature gets down to 45 degrees F (generally in the
middle of the night) the audible alarm goes off (the lights do not flash). This continues for a couple of minutes and then generally re-occurs about an hour later.
I have checked all other aspects of the general alarm system (motion and tow away) and everything functions correctly - ie sound and flashing lights. I am assuming that my problem may be with a faulty frost sensor.
Has anyone on the NG had a similar problem and can anyone tell me where the frost sensor is located.
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