The empty pen barrel could have a completely unrelated to drug use explanation or it could have been left over from past use. I know that for a few years into recovery, I kept finding empty beer cans stashed in weird places left over from drinking days.
It also could be from current use. If it is, more clear signs will show up shortly. I was never as good at hiding it as I thought I was.
Just keep doing what you are doing. Stay focused on you because you are doing good as far as I can tell. Goodatcha!