Spawning buckets are attached to Pokémon spawned via natural causes (i.e. a Spawner, as opposed to commands or direct world instantiation). This data lives on the Pokémon’s persistent data, at the tim_core:spawned_in_bucket string property, but you can also use the Pokemon::getBucket extension function from Tim Core.
If, for any reason, you would like to disable Tim Core adding these buckets to Pokémon as they spawn, change the addBucketToData config option to false.