Help please

well...took the siren off last night as not to annoy the neighbours just the pretty orange lights flashing which is quite festive....
took the car down to my sons work (trainee grease monkey) and he plugged there diagnostic machine in and it came up as a fault to the rear right hand side alarm sensor...now talking to dave this morning he advised me that it could be the "B" pillar sensor so ive ordered one and see how we go....