The HSA6200 is a simple siren-based set with no control panel, which means it has no display to show where the alert is coming from.
It should have two door (or window) sensors, and two PIRs.
Assuming that the doors are not loose and don't rattle in their frames, I'd start by taking the batteries out of the PIRs. It might be that one of them can see some hot or moving object, perhaps a halogen lamp or a mouse, or maybe it is looking at a window and can see something moving outside. If that doesn't do any good, take the batteries out of the door sensors.
If it still goes off when there are no batteries in the sensors, then either the siren case is loose on the wall and the tamper switch is operating, or there might be some other electrical or electronic object, perhaps a security light, that is causing interference. In this case bring the siren into the house and see if it happens when the siren is lying on a flat table (which presses on the tamper button) in different parts of the house. They sometimes suffer interference from a wireless internet router or a baby monitor, but I have only known that block or weaken the signal, not set off an alarm. It is theoretically possible for the "anti-jamming" circuits to set off the siren if it is getting so much interference that it thinks someone is trying to overwhelm it, but I have never spoken to anyone who has experienced that in an ordinary domestic house in an ordinary residential street, and I have never known anybody need to turn off the anti-jamming detection. If you lived next door to an arc-welding factory or a BBC transmitter, it would be troublesome.
If it turns out to be a router, baby monitor or something, you will probably be able to cure it by moving the siren and the other device further apart. It is easier to try that by moving around inside the house than by running up and down ladders to fix it on different parts of the wall. Interference may also be coming from an adjacent home. You only need to fix the siren back on the wall when you have found a safe place. You can leave the dummy box on the wall until then, so people will not know your alarm is out of action. It is preferable not to have the siren annoying the neighbours with false alarms, they will learn to ignore it.
BTW for the benefit of other readers, I strongly recommend the better HSA6400 alarm as an alternative, which includes a control panel and costs more, but is frequently sold at discount. As well as showing you the origin and status of alarms, it can dial out and call you or keyholders. Sorry.
