I've got the same problem (sort of). About a two weeks ago, my check fuel cap light turned on, but the cap was tight. After disconnecting the battery to erase the code, it came back on, except this time both the CEL and the check fuel cap lights turned on. I bought a new gas cap, put it on, and erased the codes again. Everything was fine for about a week, but then the CEL turned back on. The code was for a large EVAP leak, and I erased it again. Yesterday, the check fuel cap light turned on again (no CEL this time).
So, my problems run deeper than the fuel cap. I'm relatively sure the problem is from when I put the new fuel pump in. Anyway, your's is probably just a bad fuel cap. Try disconnecting the battery for ten minutes to clear the code, and see if the light comes back on. If it does, go get a new cap, they are about six bucks.
IT is the EVAP management system....located at the right side of the engine net to firewall9 Standing in front). It has 2 hoses and 1 wire harness. After $500 to fix it this ended up being the culpret. About $60 at Ford.....I'll bet my next paycheck on it....about $1300.