Concealed firearms are unlawful in Canada, even by plain clothes police

They're unlawful for civilians. Cops do it all the time, so do some soldiers. Those rules don't apply to military/law enforcement. It's the same reason they're allowed to possess full-auto machine guns, anti-aircraft missiles, and all kinds of other shit civilians aren't allowed to possess. The rules are different for military/law enforcement.

Another example: Civilians are not allowed have restricted firearms anywhere besides their registered storage location, an approved range or transiting directly in between. Well, here's a cop in possession of a restricted firearm in neither of those places, because the rules are different for him than for a civilian. Same goes for concealment.

Hell, as an army corporal I was once ordered to draw a 9mm from the weapons vault, conceal it under my uniform and escort the finance officer to the bank because he was dropping off a large cash deposit. Because those are the rules in the military: Any cash transfer over a certain amount requires armed escort. And there are no stupid rules about "concealment" in the military. That's civilian nonsense. You're either armed or you aren't in the military. Whether you want display it or not is up to you and your ROEs.

We ended up not going because the finance officer's schedule got fucked up, but someone else ended up having to escort him the next day, armed.

they are not legally allowed to carry carbines in normal course of duty

Says who? Honestly, where are you getting this? That's not true at all.

And they're probably not carbines in those backpacks, probably more like MP7s. Carbines would not fit in those little bags.