I used to work with the wife of a Immigration inspector (Sarnia MI). One flag he told me about was declaring nothing after a long vacation. Story - elderly couple returning from Toronto said they had not bought anything. Sent them for search, and found 200 beanie babies (collectable stuffed animals) under the spare tire. Confiscated and fined. They will also ask stupid questions or ones they have no idea the correct answer for to try and fluster you, and entertain themselves.

Very funny related story. Had a supplier in Sarnia coming across to visit one day.

Immigration Guy(IG): Where are you going?
Supplier (S): Acme company for a meeting
IG: Who are you going to see?
S: Debbie.
IG: Debbie's not in today. She's at a supplier in Ohio.
S: (Very confused) OK. I'll see tev9999.
IG: He's not in either - they are both in Ohio.
S: (Dumbfounded) Ummm?
IG: You can go. And by the way, Debbie's my wife.

Even better - a couple months later:
IG: Where are you going?
S: To see your wife
IG: (About to pull gun) - Oh, its you. Go ahead.

So maybe they do know the answers to dumb questions.