Not wanting to point fingers, but this sounds like someone with intimate knowledge of the shop and/or security system. Once, could be a lucky break for some chavs, twice that's dodgy.
S%*te I know, but I would take a long hard look at the ppl that might have this information.
My dad used to own a kitchen/bathroom shop (before he retired to Spain, lucky %*^$£ ) and was hit hard twice with break-ins.
It turned out to be the delivery driver (who knew when the expensive deliveries came) and the guy that had fitted the security at the shop (who had been a family friend ) They were caught the third time when my dad installed some 'added security'
Hope it isn't anyone you know, but experience has taught me to be very distrustful
Only advice I can give is, if you can change positions of alarms/sensors without informing anyone then do.
Good luck in the future. Don't let some *******s spoil your shop.
DP