Hello Keith,

It sounds like a designating flag for the Army of the Potomac as outlined in General Orders No. 102 (24 March 1862). A rectangular red/white/red flag would indicate the Third Brigade in the First Division of any of the AoP's Corps.

A good explanation of these flags can be found at: http://www.gideonflags.com/AVM/DESIGNATING_FLAGS.pdf

I am not sure why such a flag would turn up at the Wilderness, but I know some re-enactors have a somewhat loose understanding of flag usage.

Ryan