In Toy Story 3, Bo Peep received nothing more than a brief mention as one of the toys that had been lost between the second and third installments. We got no story as to where she went or why. The implication was that she’d been thrown away or given away, as many old toys are. It sounds, based on information released by Disney, that Bo Peep may have simply left Andy’s house on her own to go looking for adventure.
Article source: https://www.cinemablend.com/news/2465900/bo-peep-returns-in-new-toy-story-4-teaser