Drift bags and vagrants are one of the most obscure mechanics of the game. Drift bags are dropped items that are randomly transported to another player's world where they can be picked up, if they aren't picked up they get transported again. Once the drift bag has been transported enough many times, it turns into a vagrant, a rare enemy that spawns in no other way. Vagrants if undefeated will be transported to other players' worlds much like the drift bags.
source: darksouls.wikidot.com/vagrant