If Bennett died on a battlefield, especially one occupied by Federal troops after the fighting ended, he would've been buried in a trench with other Confederate dead found nearby. If he died at Fort Smith in November 1862, Bennett was probably being treated for disease or possibly wounds. Disease caused many times as many deaths as battlefield injuries, so that's the probable cause.

It sounds a bit strange, but Thomas Bennett may be buried as an unknown in the National Cemetery at Fort Smith. The VA site for Fort Smith lists four Confederate general officers among 400 or so soldiers buried there before Confederates left the area. In a town that small, it seems likely that Confederates who died in the hospital there would be buried in one place.

Confederates (former or otherwise) aren't usually allowed in the confines of Federal property like this, but since they were there first, guess it was too much trouble to 'evict' them. Here are the links --

http://www.cem.va.gov/CEM/cems/nchp/ftsmith.asp
http://www.interment.net/data/us/ar/sebastian/ftsmithnat/index.htm

It was interesting to learn that Judge Isaac Parker, the "hanging judge" at Fort Smith who figures in the film, True Grit, was a real person and a Federal war veteran.

One other possibility -- if Bennett's family lived a reasonable distance from Fort Smith, someone might have come there and brought his body home. Travel there by wagon, finding people to locate and dig up the body, and getting back home with a corpse in tow was a daunting task.

The weather in late November/early December 1862 probably would have made this even more difficult, so you be the judge as to whether or not it's likely.

Some folks did things like this....