the lettering is from CCF's own campus wayfinding. you're right it's for the patients.  https://my.clevelandclinic.org/-/scassets/files/org/patients-visitors/visitors/campus-map.ashx
Don't think you can see these letters shown on any building, but all their signage has it next to the name.

As for access details comment, this parking building has a name tag that is  "P1 - Elevators on SE Corner Pedestrian Basement...." and the other P2 building similarly has "skyway access on level 3 in the name". Thought the name tag was a weird place to put that, but I can see how it serves a function.

If we like this format with leading parentheses for the wayfinding letter, there are a few buildings I can change to make consistent.