The in-laws are on holiday and their alarm has gone off, triggering sensors 1, 3 and 5 which seems odd to me. There was no sign of an intruder so we can only assume it was a false alarm so I struggle to understand how this could have happened (ie, how three different sensors were all triggered).

So my question is, is it feasible that there are two circuits (hence 1,3 & 5 triggering) and sensors 2, 4 and 6 are on a second circuit (so the fault was triggered in the base unit which lit up 1, 3 & 5)?

Many thanks

Could be the ghost nobody told you about

Depends. Relatively rare, but there are some systems that run addressed devices on radial arms. Can’t completely rule it out without more info but it’s unlikely on a 6 zone system.
Most domestic systems will run individual wiring for each zone (you might have multiple detectors on one zone, but not know which device triggered for instance)
Do you know where the devices are?

I am not entirely sure – they have trip sensors on the doors and a few movement sensors around the house . Unfortunately I don’t know a whole load of very much about their set up at all.

However, the person who first checked (a neighbour, we live some distance away) said it was those three sensors however looking at the base unit, I wonder if he saw three red lights and assumed they corresponded to sensors when in fact it was simply a fault code (i.e. ‘three red lights means x’)

Just hoping we don’t get a midnight call as their neighbour is an old fella and isn’t up to sorting this out (he was up at 1.30am armed with a knife last night)!