Alarm activation

[Developer]

My 2004 E320CDi has begun sounding an alarm with flashing indicators when sat locked on the drive. It also appears to put on the interior lights at the same time. The alarm isn't the horn. It makes a quieter siren type sound. Can you please tell me what tones the tow away protection and interior motion sensor are? I've deactivated both at present.The only recent electrical difference is a new rear LH parking sensor fitted about a week ago.
Could it be the cold weather perhaps? Any other ideas gratefully received.

Many thanks.

 

[Cole@MBS]

See the same on a 220 cdi today, looked at last ATM source,i,e what triggered the alarm, turned out to be the tail gate latch, cure, new boot/tail gate latch!