Evening 

My alarm keeps going off (seemingly, only at night) and I'm struggling to find help on the cause. 

I've previously had the issue with the rear washer jet hose popping off, giving me the boot open warning and the hazards not flashing when trying to lock the car which I sorted. 

However, this time everything in the boot is dry, the pipe isn't leaking, I'm getting no warning about the boot being open and the hazards flash when I lock it but its started. 

I did find a couple of threads about it being caused by the battery starting to die but they all get hijacked by people talking about water in the boot 🤣

Battery reading between 13.6-13.8v when I'd expect it to be low 14s, so I've put it on my reconditioner overnight to see if I can get it up to where it should be, but if this doesn't work, what else could it be? 

The static battery voltage is meaningless when trying to diagnose electrical faults. Seemingly good batteries can actually be defective and cause a variety of problems. A deep discharge test, which most can not do at home, will reveal any fault.

Having said that, there could still be a residual fault from the leak. The boot may now be dry but have you checked for moisture or corrosion inside the lid switch? I also seem to recall reports here of damaged wires to the boot lid where they pass through the rubber grommet causing a similar problem.

Yep checked all the wires for corrosion and was pretty much nothing to see. 

I'll see how battery comes on and it it's still iffy try a new one 

Was the issue with the battery then?

It turned out the back window reg had failed so the window was ever so slightly down, meaning a draft was getting in and disturbing the air freshener round the mirror, which then set the motion sensors off 🙈

Lol

how on earth did you find that out?! 
 

Did you ever do a diagnostic test?

Realised the rear passenger window had dropped when driving the car and heard the noise. 

Alarm was only going off in bad weather. Removed the air freshener after reading these sometimes set car alarms off, alarm stopped going off. 

Got window sorted and alarm never gone off since