The latest is a warning sign featuring "No Weapons Or Alien Tech Beyond This Point" (found below).
This comes to us from executive producer and writer Maurissa Tancharoen via Twitter.
We know that the aliens, the Chitauri from The Avengers movie, left advanced weapons behind as there was the "Item 47" extra on the Blu-Ray (pictured right).
The sign below seems to suggest that this might be a facility (airport?) where S.H.I.E.L.D. Agents, or maybe anyone, can't bring anything alien through.
So it could be that the agents of S.H.I.E.L.D are outfitted with advanced alien technology to battle whoever or whatever they are up against.
S.H.I.E.L.D. looks to debut this Fall with the pilot written by Joss Whedon.