The NASA HQ administration building off 4th and E just south of the Mall has no armed guards posted outside, and there's even a NASA public credit union in the building, which does not require passing through the metal detectors or security post just a couple doors away in the same open lobby.

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.

The NSA (National Security Agency) and NASA are very different organizations within government.

I misread the OP as NASA, which was careless but I still think the observation is worth noting. Also worth noting is that there is a museum on the NSA campus (National Cryptologic Museum) that anyone can visit without any clearance, and though there are vastly trickier chicanes to contend with compared to the access at NASA administrative HQ, merely visiting the cryptologic museum ushers one past several otherwise highly restricted perimeter zones.

Yes, it is understandable that security works that way in a building occupied by people whose job it is to keep secrets... but that is not the way security should work at a building of democratic representatives where their job is to be publicly accountable.