This was a drift item bag. When another player drops an item and abandons it, it has a chance of being sent to your world. It can be sent to you if you're hollow, and in non-multiplayer areas as well.
If you don't pick it up and abandon it, it will also get sent to another player's world again. If this happens enough, it'll eventually turn into a Good Vagrant. (I currently have no idea how many times this has to happen, or if a certain amount of items greater than 1 also need to be passed simultaneously).
I don't know of all of the possible items that can appear in them, but Lloyd's Talismans, Rubbish, and Prism Stones are the three most common things to be found in them. I've also found a couple titanite Shards as well, making those the only current 4 items I'm aware of. Items you drop turn into these items when they arrive in others' worlds, meaning these aren't necessarily what they dropped.
So it could be something like dropped Purging Stones will get passed around to other worlds' as Rubbish, and then it eventually turns into a Good Vagrant that drops a Transient Curse. The item you find (as of now) doesn't seem to tell you anything about what was originally dropped. I also don't yet know if there's any significance or difference between finding one of the four known Drift Item bags yet.
As tempting as it is to pick up a Drift Item bag to see what's inside, it might be better to leave it be. You're preventing a Good Vagrant from being born.
source: An educated guess based on a lot of testing, where I've learned how to spawn these stray item bags on purpose, as well as create Good Vagrants for myself:
Vagrant TestingI'll need to conduct more tests, with the help of modders/ hackers on the PC, to find out what's really going on behind the scenes and learn more about the details of this obscure mechanic.
edit:
titanite Shard
titanite Shard
Weird, the board is automatically changing the spelling of "T-itanite" (with a capital T), to "titanite".