My one,and only,"experience" with Canada and firearms occurred about 10 years ago.Driving from North Dakota into Manitoba I obviously had to be cleared by Immigration and Customs.For some reason they sent me to "secondary"...the very first time I had ever had a lick of trouble entering a foreign country.The facility was basically deserted except for me and a pickup truck with Alaska plates.
For about 90 minutes they proceeded to grill me about firearms while they pulled apart my car.Question,after question,after question.They even demanded the password to my laptop and told me that if I didn't provide it they'd seize it.
Mind you,they were always civil..even polite...but they surely let me know that they were in charge and that they had something on their mind.
After 90 minutes they let me enter..and I've never had a problem since (I've entered Canada about a dozen times since).
Given that I have a very common name the only think I can think of is that a guy...an American...with my name was on some kind of watch list.
My wife speaks of the time when she and her ex husband drove to AK about 25 years ago. Their moving van was completely unloaded and all content was thoroughly looked at.
The Canadian Border patrol has very wide powers of search, seizure and arrest.
They DO NOT need to have probable cause.
They don’t like the kind of sunglasses you wear? They can tear your vehicle apart piece by piece and make you pay to have it put back together.
And they do not even need to justify tearing your car apart by finding anything.
Always cooperate. If you need an attorney at any point, call one and say nothing.
You were likely simply randomly searched. They do that to every nukbered cout of vehicles , decided before hand.
Best time to cross is when they are very busy.Open all your windows weather permitting so they can see inside, Take off yout gat an sunglasses so they can ID you, and “sir” or “mam” them, looking them straight in the eye when you answer questions. That wll get you by most of the time.