I've been watching a TV show about border security in Canada and in the episode a border officer mentions that posting an "owner is armed" sign inside your vehicle is illegal. Is this true and if so, what law is being violated in this case?
I believe the appropriate snippet is about 10:59 and 11;35, and no one accused him of violating any law - they were just "playing it up" for the TV. In fact if anything the opposite, as they did not confiscate the sign.
At 11:45 the officer explains her rationale - effectively a sign like that could put an officer of the law on high alert as its outside the country norms - increasing the risk of the encounter for all involved.
I am not familiar with Canadian law per-se, but, like pretty much all jurisdictions, if one looks hard enough one can always find a law which can be stretched to justify the accusation of a crime. Intimidation? Threat of assault? - again though, he was not accused of any crime - and almost certainly this was just "done for TV", like the rest of these shows.