At the game shop we were unable to find an answer. Mindwarper says, "ACTION: Choose and enemy creature. It captures 1 {aember} from its own side." This creates a (potentially) odd situation where, on a single creature, some aember is from one player and some aember from the other player.

My assumption is that when aember is placed on a creature it has no "memory" from where it came. However, others felt like when the creature (with its own team's aember) died, the amber should return to the original owner NOT to the opponent.

Personally would go for simplicity and the glossary definition of capture. Captured Aember goes to the opponent. Keeping track of aember sources would just add a complication into the game.

Always take the rules literally. If it says return captured aember to the opponent, it goes to the opponent, regardless where it came from.

The whole point of Mindwarper is to steal æmber, it just does it in a round about way.

Doesn’t matter where it came from, when a creature owned by one player dies with captured amber the amber goes to the opponent of the owner no matter where that captured amber came from.

Captured amber always* goes to the opponent of the creature's controller when it is removed from play (dead, archived, purged).

*Barring a card that breaks this normal rule of course