First Dana produces this. Then Ross produces this. Follow the links back in Ross's posting. It seems there are some questions as to the identity and purpose of some of the protesters at Montebello. Canadian Press has the story here.
Update: Canadian Cynic and Red Tory have more. The question of the "yellow triangles" on the bottom of boots, (police boots matching those of protesters) means nothing. Big City Lib identifies the culprit as a standard brand of outsoles found on many different brands of boots. In fact, I checked the shoe/boot supply here and found the same marking on the bottom of a pair of Columbia® hiking boots and Hi-Tec® hiking boots.
Shot from the sideline: Way back in the day when all of us young fresh-faced baby-servicemen were learning the finer points of "Aid to the civil power", otherwise known as riot control and riot suppression in support of a local police force which found itself outnumbered, I recall an old training film. It was made sometime back in the early 1950s and described the stages of a peaceful protest turning into a violent riot. Now, for what it's worth, it was British-made and was filmed in Malta. There was, throughout the thing, an air of British imperial rule, but that is perhaps beside the point. The point is that even back then, there was a description of the "local constabulary" using "native locals" within the crowd to "monitor" the behaviour and identify "ringleaders".
There's nothing wrong with that. It happens at most protest gatherings. As long as it's just looking and gathering information. However, even back in the days of black and white, there was a line in that movie which actually prompted some immediate questions. "Agents for the police in the crowd may take action in order to conceal their identity from other rioters."