T.I. BACK IN FEDERAL CUSTODY

TMZ has learned ... the reason T.I. was hauled back to prison from his halfway house is that he allegedly misled prison officials into thinking he'd take a low-key mode of transportation from the prison to the halfway house -- and a tricked out tour bus isn't low-key.

Law enforcement sources tell TMZ -- when a prisoner is incarcerated in a low or medium security prison ... prison officials can allow the inmate to use a private, unescorted mode of transportation to travel from the prison facility to the halfway house.

During the process of approving such a transfer, the prisoner must state specifically what mode of transportation he or she will be using.  Our sources tell us ... T.I. informed prison officials he would be taking some sort of van, but made no mention of the high-end, pimped out tour bus