As it happened a couple of months ago when Season Passes were available for free on the PlayStation Store, Fallout 4 has recently been available for free on the Xbox Store, and that includes the Season Pass and all DLCs.
At first some players thought that it was a special promotion, but unfortunately for them their hopes were dashed shortly thereafter when it turned out that it was simply a mistake. Microsoft apologised to players and confirmed that they have already revoked the licenses for those that grabbed a copy of the digital game thanks to the bug.