Very hard. Lupin picks and chooses his targets mostly, and has certain people he watches. He has a plan, you see, and he has certain people he would trust with the plan.

And no, it wasn't poison itself. It was pain medication for Lupin's broken ribs combined with alcohol combined with sleeping pills. YEEAAAHH...Lupin passed out.

Also, I'll pm you later, but Robin would know something Lupin will try to keep quiet in the coming weeks and months.

(he'd also know about the shenanigans that went on during the first AU shift with Adele Leblanc in his room)