It goes without saying that bringing things back into the US in carry on luggage is not a good idea. I fly every couple of months and there is decent chance they will look through your carry on (even domestically).

Fortunately, they are looking for other stuff, so they could overlook something.

The odds of them opening your checked baggage are lower. More often then not they go through the xray machine. You're always taking a chance so better to keep quantities small.

One last thing - never put a lock on your checked luggage. It is now against airline policy (and perhaps US law??) to lock it. If you do it's a red flag.