Originally Posted by law dawg

If it's been dropped in the mail just that day then you're right - I can't prove it (of course it says that travelers are supposed to file the form with Customs at the port of entry or departure....). I'd would detain you long enough for questioning and notification to CBP. I'd ask things like what kind of questions were on the form, how many pages was it, etc. Things you should know if you really filled it out. Then I'd watch to see how nervous you were, other little tells. If I believed you, off you'd go. Ten minutes tops. If I didn't, off you'd go still but with me forwarding all the information I had to the appropriate agencies.