Whoever got to it before you might not have looked inside. Grab that shit and hope no one saw what was in there. If no one says anything then keep your mouth shut. Remember, just stick to the lie until you know for sure that they know you are lying. If they ask what it was make some shit up. You may get in trouble for buying weed AND lying, but there's a chance they might not find out at all, and I'm a risk taker, so I'd go with the full bullshit route.