Nel has lost the plot. Does not know left from right... and must be the ONLY person who does not know that OP referenced the earlier paint job(2010) where sensors were removed... that was WHY he thought they MIGHT have been removed during the renovations at the time of the incident.
OP is not explaining it well... but Nel is also NOT understanding that you can take a sensor off the wall... it will still be "working" where you store it.. and not show as a fault when you activate the system... but it is still not on the wall sensing intruders as it were.