I will defend teh mapmaker. My guess would be that the doorway is well-defined, and that there is a sidewalk leading towards the road. If the cross is in the middle of the doorway (even if it is offset, it will be easy to position in relation to the center), then the Start/Finish will be in the center of the sidewalk.
As such, that is an easy location to identify. Paint, and even nails, will disappear over time. The first or second time the street is plowed (for snow, as it is Chicago), the nail could be removed.
Parking signs, street signs, and even telephone poles are removed often enough to allow the cross to be considered as permanent as a sign post. Now, if there were a fire hydrant handy...
If the Start/Finish is mis-judged by even a meter, that is still well within the SCPF. No harm, no foul, in my opinon.