It wouldn't even have to have been an airport. A small ultra-light or similar type plane could take off from any relatively flat strip of land. Heck, they could have even taken off on a flat stretch of roadway. I'm sure there are plenty of smaller such roads and areas in more isolated regions of the country.
If more than one person either saw and/or heard a plane going over the depot right before the explosion, it does seem logical to conclude that they MIGHT have been right, not just dismiss their claims because there is no log of a plane taking off or landing at the nearby airport.
I just have this nagging distrust of government statements, that's the way I am and I can't change at this late date. I also have a nagging distrust of Islamic terrorists.
Additional note - I live about 4 miles (maybe 3 as the crow flies) from a very small local airport. I've seen small planes take off and land (maybe 4 seaters at the largest) and had them fly directly overhead my property many times. Sometimes in the summer ultralights fly around too. I always hear them if I am outside, and anything larger than an ultralight I can usually hear inside the house unless I have some other loud sound going on. So the sound of even a small plane is not insignificant and is rather noticeable.
http://www.freerepublic.com/focus/f-news/1538569/posts
Firecracker sparks blaze on wedding bus, killing scores
Interesting article. An accidental explosion, caused by firecracker meeting gas tank.
So, if the explosion/s at the fuel depot were caused by gas leakage meeting some kind of spark or fire, it could just as easily be accidental as a caused act. If it was terrorism, nothing could be cheaper or easier.