Okay wait a minute. I don't think roxi planted it I think it's Adam's partner that cleans up his messy kills. Because he was in the phone with someone telling them "You know I hate killing men. They are always boring and never satisfy my needs." Then he hangs up the call while looking at the women standing outside as he said nevermind I found my new project. I think it's his partner that left the shirt in the car on Purpose by working with the cops or they just forgot to grab it. I mean you never know.