Virginia,
It is most likely that your relative WAS in Corinth at the time of the battle. On the 1st of October the Confederate Army of West Tennessee was astride the Memphis & Charleston Railroad at Pocahantas, Tennessee, cutting off communication between Corinth and Memphis. On the morning of the 3rd of October a squadron of Brigadier General Frank Armstrong's cavalry cut the Mobile & Ohio Railroad, thus severing the direct route north to Jackson, Tennessee.
By the last of September it was a well known fact that the Confederate army was on the move from Ripley, Mississippi northward toward Tennessee. Union Generals Grant and Rosecrans were moving troops around the chessboard of northeast Mississippi and west Tennessee in response to the Confederate offensive. Your relative had a very limited number of options to exit Corinth prior to the battle. It might have been possible to get on a train leaving for Jackson on the afternoon of the 2nd or he could have gone overland 23 miles to catch a riverboat at Pittsburg Landing. Your relative was in the Regulars for five years and probably had the savvy not to attempt travel when the enemy was on the march in the neighborhood.
Tom Parson
Park Ranger
Corinth Civil War Interpretive Center