The only person that can forcibly stop and search you is a police officer* and only then if they have reasonable grounds to suspect that you have some form of prohibited item on you: for example, stolen property, drugs, weapons etc. If you refuse to allow a police officer to search you when he is lawfully doing so the police officer may use reasonable force to undertake the search and if you obstruct them in the search you commit a specific criminal offence.

That said, anyone can walk up to anyone else and say "can I have a quick look in your bag please and you may, of course, allow them to. Interestingly, police officers do not have the power to undertake a voluntary search - it's either got to be a statutory power or nothing. If you refuse to allow a security guard or whoever not to look in your bag then they are not allowed to use any force to compel you to and you would not have broken the law. However, any premises can make it a submitting to a search condition of allowing you on the premises or continuing to remain on the premises. Examples of this might be nightclubs that insist on search before entry. Even in these cases the search can be refused and force cannot be used to effect a search - the typical net result of a refusal would be that your invitation to remain on the premises or continue your business would be revoked and you would be asked to leave. If you refused to leave then the agent of the landlord of the premises where you are can use reasonable force to remove you as you would be a trespasser.

In this specific example - where a door alarm etc goes off - that, of itself, would not even provide grounds for a stop and search even if a police officer was present. A security guard can use reasonable force to detain someone pending the arrival of the police if that person has stolen something. The guard would have to have rather better grounds than a refusal to have your bag searched.

In terms of what I would do if challenged it would really all depend on the attitude of the security guard speaking to me. If I hadn't taken anything (which obviously I wouldn't) and the person was being an idiot then I'd, personally, just walk off. If they then tried to physically do anything they'd be finding themselves in a police-cell pretty sharpish. I'd advise to tell them in a clear, loud, voice to stop what they are doing, ask them if they are detaining me (hopefully whilst recording on my phone) and if they said yes then I'd make a complaint regarding unlawful imprisonment (or kidnap if they insisted on taking me to a back office).

There is a final - rather more helpful point to make - in that if alarms are going off because the cashier hasn't taken the security tag off of your bottle of Whisky then it might be useful to allow it to be taken off so you can enjoy it when you get home!


*except in some special circumstances such as prison officers in prisons etc.