Shutting Off Interior Ultrasonic sensors (not in manual)

I think I’ve accidentally come across another way to shut off the interior ultrasonic alarm sensors (useful for ferry crossings, dogs in car etc) however it may also be switching off doors and bonnet alarm as well!. If you step out the car and lock it as usual, the immobiliser activates as does the rest of the main alarm (Sigma?) I get the usual flashing immobiliser  dash light and the second small LED red light (green/amber/red). If I then walk round and open the boot via the touch pad above the number plate, boot opens, the doors remain locked but the sigma interior ultrasonic alarm deactivates (and red LED light goes off). If I then press the close button on the boot lid (but not the close and lock button) then the boot closes and everything else remains locked but the red alarm LED doesn’t come back on. The immobiliser flashing light on dash stays on as usual. So I’m guessing at the very least the ultrasonic sensors are off, but I suspect the rest of the Sigma alarm system may also be off doors, bonnet, boot. Has anybody else come across this, and does doing this mean I’ve deactivated the full main alarm (except immobiliser) or just ultrasonic sensors? May a quick and easy way to switch off alarm in certain circumstances while keeping car locked.