You can wave any object at the sensor. Maybe an unauthorized tag will yield a different beep or make the light flash a different color. Maybe the person in front of you will be in a position to see the light on the reader, maybe they'll notice, and maybe they'll consider it odd. Getting that far, and then actually deciding to challenge you or report it, is a vanishingly small chance.

There is no point in badging an unlocked door, or in expecting people to do so. You have to actually close it between entries. This is a physically and socially ridiculous thing to do with traditional doors; if it's what you want, you need a turnstile.