That's why in the UK, it's wise to simply tell them to get lost - if they lay a finger on you without justification (ie, you haven't nicked anything) - then you can easily get them for unlawful arrest. Tesco have been caught a few times by this there.
No, security in a retailer's premises in the England and Walescan
lawfully briefly detain you for suspected shoplifting, that's the law. They have to call the police, however, if they detain you. They have a special room for it in most shops. Otherwise every shoplifter would just leg it.
Basically, you are entitled to detain a person that you suspect of shoplifting, as long as you have reasonable grounds for this suspicion. In order to detain them, you are also entitled to use a 'reasonable' amount of force. Naturally, if you forcefully detain someone and it turns out that either they weren't shoplifting or you can't prove that they were, they may then be able to accuse you of assault.
For this reason, you should make sure as far as possible that you have reasonable grounds to believe the person is shoplifting before you even approach them.
The alarm is, AFAIK, considered reasonable grounds. But I could be wrong. That was what I understand as reasonable grounds - don't shoot the messenger.
If shoplifting is established, many shops then opt for a banning order on the shoplifter, but some go to FPN or court etc.
Where there's an innocent mistake, such as in the post above, not much you can do really except hope they have the decency to apologise. I've never heard of them giving vouchers out as a sorry etc.