mystery bag

noun6

1. •
   A bag whose contents are unknown.

   • I drove home with the mystery bag on the seat beside me. I was going to do as he asked and wait until I got home to open it. — 2010
2. •
   A bagged package, randomly picked, which contains surprises (e.g., mystery prizes).
3. •
   A bag from which a gift may be withdrawn (usually unseen until after chosen).
4. •
   (Australia, slang) A sausage.

   • Bully beef and spuds, tripe, fish′n chips, Anzac bikkies, damper with cocky′s joy (golden syrup), snags (or mystery bags) and hot custard and jelly for sweets. — 2010
5. •
   (Australia, slang) A pie.
6. •
   A non-specific mixed lot of a product at the supplier's choice.