How do you notice someone disappearing in an alley, exactly?
Alleys have the tendency to always have one curtaintwitcher/local drunk/People of Bad Repute hanging around.
Even in this modern day and age it's the alleys/courtyards/side streets where things
really happen. Not ever in the main streets.
So there'll be
someone who noticed the "going in, but never coming out" bit involved here.
The art is, of course, to know who these people are, and getting those people to talk to you so you know what's really happening......