@yuchiyo I was going to say this in an earlier episode, but I suspect that the chocolate that was in the table (one that looked like bitten?) wasn't probably put by Yoo-il, either, if that's the case. Someone also pointed it out in the top comments of that chapter in Naver, too. Still, too subtle (AND WAY CREEPY) for the stalkers, if that's the case...
In general, I think this episode fully confirms that it's not about "one" stalker, but a group of stalkers, actually. One had a tattoo in their ankle, it was previously mentioned about one in a wrist? and the one in the back of a neck (the one in this chapter). What do they want from Moo-yi, though?