When you use the Purchase Order's "Unissue" button, the PO form automatically closes although you probably intended to make changes immediately.  So you don't have to find it again, it's recommended to not select any other PO after you open the PO intended for the unissue.

Step-by-step guide

Recommended PO unissue workflow

  1. Go to the Purchases module
  2. Double-click the PO you'd like to unissue
  3. Immediately click "Unissue" (here is where it auto-closes)
  4. Immediately press Enter to reload the same PO (or just double-click the already-selected PO).