You knew about DawkTrap. He would possess Lewis every now and then, blah blah blah. The thing is, you didn't trust DawkTrap alone in your house, so you brought him with you as you went to the store.

"This is stupid, I'm not a damn chihuahua."

DawkTrap sneered, pissed off by your plan. You attracted a leash to the back of Lewis's backpack and made DawkTrap wear the bag. Mostly so he wouldn't bite anyone that looked at you, Lewis's partner.