the Bank of Canada will take them out of circulation if they find them

No they won't. In fact, if banks send in marked notes to the BofC that are in otherwise good condition, the banks are charged by the BofC to put it back into circulation. Only if the note meets BofC standard removal criteria is the note destroyed.

That being said, I have been told by some tellers that their branch will purposely mutilate a marked note (a Federal crime!) to get it out of circulation.