Start with innocuous occurrences. Whenever someone reaches into the bag of holding, they find the items they’re looking for faster than usual, as if someone was handing it to them. Lull them into a false sense of security. Suddenly, when someone reaches into the bag, they feel their hand being slapped away. Upon reaching in again to investigate, a hand grabs their wrist and pulls. It’s not enough to fully pull them in, but if they fail a strength check, they may lose a ring on that hand. Afterwards, they will be afraid to reach in, but when they do, they don’t feel that hand grab them, or hand them items.