Having spent time in FEMA training facilities (work related stuff) I can't speak to the meaning of the markings but I wouldn't be too concerned about them having nefarious meanings. Most of the FEMA peeps I know are nice guys and gals who have a sense of humor about stuff. Once I asked one of them about "FEMA Camps" and if they existed or not. He chuckled and in a semi-serious tone said "You'll see." We all had a good laugh about that one.
This is hearsay on my part, but another colleague was at the same training facility at a different time than me and "supposedly" overhead a conversation between two FEMA guys who were talking about UFOs...and not in the sense that there are lights in the sky that we don't know what they are but as in spaceships with little green men. Whether this is true or not or he's pulling my leg I don't know. Interpret it as you may.