Get a lawyer.

If your car didn't come with a front mount already; meaning you'd have to drill holes into your bumper, you might get away with it as long as you had the plate in the car with you.

Also, you can always just say it fell off the other day and haven't had a chance to put it back on.

Again; get a lawyer.

Until I read those two links I was under the impression that you didn't have to display the front plate unless there were current mounts on the car; meaning you didn't have to mod anything to mount it.