Peugeot 'alarm'

[baggyoops]

My Peugeot 306 makes a strange whining warning sound often when I start it up late at night. At first I thought it was an ice warning as I first heard it when the temperature was around freezing. But it does it in summer too. It seems to be coming from the sensor in the roof above the windscreen and the only thing I can think of is that it comes on when the temperature drops a certain number of degrees. It bears no relevance to rain or whether the lights are on/doors are shut/seatbelts are on . .. . does anyone else's 306 do the same? I can't imagine ours is the only one.

Thanks