Never heard of a 'box' check in; 'oversize', yes.
You absolutely MUST know what's in your baggage.
We were once asked to take presents on a trip and they duly arrived all gift wrapped.
We unwrapped and examined all of them, not so much because we distrusted the person for whom we were taking them but:
a. We didn't know if they had wrapped them themselves.
b. Those security questions are there for a reason and if it becomes clear that you don't know what you are carrying then stand by to miss your departure.