What is it (today)? Nothing.
What was it?
Edit: Changed my mind and decided that as you mention in the question, this was a Unified S-Band station. This was an Apollo-unique communications system. What we knew in Shuttle as the MILA tracking station is located in this spot on the map.
The Wikipedia article on MILA states that it had
Two 30-foot (9.1 m) S-band dish antennas.
And this old "Tracking Apollo" document shows a table of tracking stations on page 107. It lists
Cape Kennedy (Merritt Island) USBS Dual 30'
where earlier in the document USBS is explained as Unified S-Band System.
The map in the question is old, it predates the construction of Space Commerce road, which runs behind the Visitor Center and allows one to bypass the NASA Parkway / Kennedy Parkway intersection. It also predates the moving of Gate 3 east of the Visitor Center and Gate 2 north of the Space Commerce intersection.
This Google Maps image shows the visitor center, Space Commerce road, the former MILA site, and an abandoned facility. I originally thought the USB on the map referred to the abandoned facility because of the way the road is drawn on the map in the question, but it seems clear to me now that it was MILA.