Sure Betting question

How does the book actually know your sure betting? If you keep your bets low enough do you stay under the radar?

No. you will not stay under any radar.
Sure betting has at least one value bet component out of the two or three bets. They can detect those bets.
Eventually you will have to balance your bankrolls between bookies. That means doing a W&D dance… A few of those and bookies will not be happy singers…