Interesting, are you sure about this? The only clause I find in the constitution mentions post offices and post roads (perhaps that's what you're thinking of?), but nothing about their security. To my knowledge federal agencies were only established by the Administrative Procedures Act after WWII (which is basically the "constitution" of federal agencies), but I'm not sure if there's other similar terminology that might be in the constitution. If someone has a link to the clause that would be interesting.