Alarms are no grounds to stop anyone, believe it or not. To make a stop a security guard has to have followed a procedure called "SCONE", which is Select - they see you pick up an item, Conceal or Carry - hide it or take it away with you, Observe - the guard can't take his eyes off you for one second otherwise there is no stop, None payment - you go past the checkouts without paying, Exit - you leave the store. Without all that there can't be any stop, they might ask for a receipt if the alarm goes off but if you just carried on walking they don't stop you if they don't have SCONE. A lot of store managers don't understand this and complain about guards for not stopping people when alarms go off, without realising that it would be a false arrest if they don't have the procedure on the suspect. When the alarm goes off inexplicably it could be your shoes, a lot of shoe shops install tags inside the soles and they are supposed to break when you get walking but often they don't.
UK Answer, if anyone says this is wrong they are American or failed to do a simple search.