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.

We're told prison officials felt T.I. should have fully disclosed the nature of his party bus.

And get this ... law enforcement sources tell us ... when the U.S. Marshals arrived at the halfway house to take T.I. back to prison, the halfway house honcho actually got into an argument with them, saying it was ridiculous to send him back for such an alleged violation. But the Marshals had their orders, and took T.I. back to the pokey.