[Festus2]

arwenmark -
Is there any possibility that the "alarm" you hear may be coming from the clock in the stereo? There may be a clock alarm that is somehow being triggered by "something". Have you managed to isolate where the alarm sound is coming from?

In a previous post, you described the sound as being a loud beep like a "fire alarm". Can you be a bit more specific? Is it similar to the loud beep of a smoke detector? High pitched? Low pitched?

If it isn't any of the detectors, I can't think of any kind of "alarm" that behaves in the way you described.

I am wondering if the previous owner may have put in some kind of motion sensor alarm that goes off when you open the door or move about?

It is a puzzle for sure.