For national security buildings (e.g. the NSA) it is the exact same as your foreign country experience. The guards around the perimeter are very quick to engage and ask what you are doing if you meander around the outside.
It is visited by plenty of non-NASA, non-government ordinary customers who could easily blend in with official foot traffic on a busy day and make just a dozen paces to the main elevator banks or stairwells looking for an opportunity.