What is this quest marker for, and how do I get there?

Solution 1:

That's a bug in DE:HR, it happened to me, too. One of the gang members has wandered off to some point where you can't reach him. I would point you to a specific website, but none of the suggestions I found helped directly.

• Some solutions are here in the Deus Ex wiki
• Or here in the Steam forum

Otherwise, google for deus ex cloak and dagger bug and try out a few things.

I opted for the possibility that the gang member is one another roof somewhere and tried to shoot him (without seeing him), but that didn't work. All out of a sudden, the marker started moving and he was back in the flat, so I could finish him off.