Directly to your question - they don't know - unless -> you call attention to yourself by either bragging about it or doing something that catches their attention - or you have an emergency
somewhere along the journey that requires assistance.
- - Then there is the chance that you will be stopped and boarded for a "courtesy inspection" on your way back. All of this is entered into the "massive computer files" CBP and DHS use.
- - The penalties are rather harsh if you are caught which is the biggest incentive to "play by the rules."
- - Getting a Local Boaters Option is not all that difficult and eliminates a lot of the hassles.