Toronto police in particular are infamous for aggressively collecting contact cards AND sharing that data with the US border. Some other jurisdictions collect less information, and some refuse to share it outside their own systems, but Toronto for whatever reason shares the most information with the US border guards - including the contents of all the contact cards.
Aside from all the C36 angles, there are a zillion reasons why Toronto should not be doing this, and this is actually one of those issues you can go bother your city councillor about without outing yourself as a participant in the sex industry. Toronto's sharing of information on ALL types of contacts has caused people problems at the border, and it is within the power of Council to order the police department to stop.
icespot: I don't think anyone objects to Toronto police using that information to stop crimes locally, but it's the broad sharing of that information that is so damaging. It should not be released on background checks in general, whether border crossing checks, or job applications. And yes you can file a freedom of information request to see what is on your file -- BUT, then next week you're detained and then released by the police in connection with prostitution. Now it's on your record, police will NOT remove it, and WILL share that data with the US border, even though they never even charged you with anything.
To anyone who winds up questioned about such a card by a border official -- DENY, DENY, DENY. A contact card is not a criminal conviction, it will cause them to ask questions but in and of itself cannot be used as a reason to deny your entry to the US. What it will cause is a further investigation into your background and a whole lot of probing questions. Only if they find something material, or IF YOU ADMIT SOMETHING, can you be denied entry. So deny. The police must have confused you with someone else, you were just walking through the area, etc.