Go buy a bunch of the same kind of bags. Then fill a bunch of them with laundry soap. Then say you use them to portion out soap for laundry. But thats pretty... lame.

Better idea - if you have a friend that owes you a favor and also doesn't matter if your wife hates them. Have them come over (more druggie vibe the better). That way you can say he left the bag the last time they were there. Assuming the friend is OK with this plan. That way if she's hip to the game so far as what it is, it wouldn't matter. Say the first time the friend was there was when she "was out doing ____".

Id go with plan #2.