I reckon she was one of your stolen ones, and now no longer needed dumped back at the allotments.
I sold some sheep and about 2 years later went to buy them back again (they had bred loads of lambs for him by then) Standing in the middle of an orchard with about 30 or so sheep milling wildly about, I did my sheepy-sheepy call, and my original 8 girls stopped and turned to the centre to start walking towards me.
I don't believe in anthropomorphising animals, but neither do I underestimate their abilities and memory.
All the best
and enjoy having her back
Sue