I would suggest that there are some significant differences that vitiate my arguments against the civil search and seizure of cigarettes when applied to your example:

1 – You would have reasonable suspicion, as the alarm went off. Whether that counts or not as being reasonable (there is some literature suggesting it may not cross a threshold to reasonability) is an argument that would need to be settled. In the cigarettes example, there is no single factor pointing to a given individual suggesting that they specifically have done something wrong.

2 – The other part of reasonability for the search is that you are looking for evidence of an offence having been committed (assuming that crossing the point where the alarms are set up is sufficient to call it theft). In the cigarettes example, even if there is a reason to suspect a given individual of possessing cigarettes, the thing suspected is not itself a crime.

Since theft is a crime, it shifts toward citizen’s arrest (Criminal Code s.494). The issue then becomes this: Is an alarm going off sufficient reason to warrant an arrest? If not, is it at least sufficient to warrant a search, which at common law is a form of detention and trespass akin to an arrest? As I said before, there are arguments on both sides of this.

Perhaps the question can even be re-framed thus:

Does the person who set off the alarm have to stop? Probably not.

If they do, is that consent to the search? Possibly.

If they don’t, do you have justification under s.494 to pursue and arrest (which would be needed to force the search)? I doubt it, as you’ve not actually witnessed the commission of a crime or a pursuit by an officer of the law; you’ve only witnessed an alarm going off as a person walked through a detector, which is not itself a crime.

I’ve written this off the top of my head, though I’ve thought about it for a few days. I wouldn’t take it as legal advice, obviously, but I hope it does further your thoughts on the issue a bit.

-Jeremy

I’m curious about a semi-related topic:
For the most part, library staff members do not search patrons for library materials when the gate alarms go off. Sometimes, we’ll ask a patron if they’ve forgotten to check something out, but we don’t tend to physically search people or go through their bags because of patron privacy, civil liberties, and, quite frankly, the hassle for us and the embarrassment for the patron. However, what would happen if someone at the library DID search a patron, without finding stolen materials, and the library (or staff member) were taken to court? Do you have any sense of what that would look like? (I certainly don’t expect you to go through QuickLaw or anything; I’m just curious about your learned take on the matter.)