One of your Flickr photos shows the concourse at Cherry St. The map is correct, showing only a ventilator in the 1400 block of Cherry St. and a wider space under Broad St., no stairs were ever built. That particular sign may have been on a side that was walled off for a storeroom while the concourse was still open for public use.
Sorry to be late to the party. Here are some images I took in 2008 of the northeast section of the concourse that curves encircles city hall.
https://imgur.com/a/GLiofOe