I believe I have answered my own question:
I made another trip to Slidell Friday. The day before, I noticed the sign was gone, but it wasn't really "gone"....
When I went to Slidell, I took a closer look at these signs that read: "LOUISIANA buckle up...it's the law". There are a few of these along I-20 only on the east bound side. These signs are actually fold-up signs that give this message for detouring I-20 in case something goes wrong with the MS River bridge. In fact, something did go wrong with the bridge a few years ago with the bridge. I don't remember the name of this particular bridge piece, but it failed and I-20 was detoured (IIRC the story that I was told) to Natchez and Greenville. The National Guard even had to airlift the piece to the bridge. I guess these signs are for "just in case" that or anything else (barge running into a bridge pier, e.g.) happens again.
I guess the bad weather we had Tuesday night must've blown the sign open. My bad.