I am talking about pieces, not parcels. But logistics is also the key point in Undefended Pieces Scan, because scan is only one part of the story. Maybe even more important is the problem to store what you have found. This is why UPS in chess also needs a sort of magazine and a clever way to stack the goods there.
A large part of UPS is bookkeeping. In the opening, there is a number of undefended pieces that emerge and disappear in a typical pattern. The basic UPS patterns are seen at certain home squares of pieces and pawns.
For instance, at the beginning, all four rooks are undefended. Typically, Rh1 is the first one to get safe, followed by Rh8. This is the case if both sides castle kingside. The next one is Ra1, and in most games Rh8 is the last one, at the beginning of the middlegame. After sixteen moves, undefended rooks are the exception. Of course, no castlings or long castlings may lead to a very different sequence.
The notorious weak points f7 and f2 play a role only in open games. Normally, f2 is saved very quickly whereas f7, if present, often remains for five or even ten moves.
The four knight's pawns become undefended after the bishops leave their homes. The g-pawns are re-defended very early if both sides castle kingside. Therefore, the focus should be on the b-pawns. The time-window where they are undefended is only about four or five moves on average. But this normally happens late in the opening, when forces are well-developed and these pawns often can be attacked. But be careful: b-pawns are often poisoned for a queen!
In open games, center pawns get undefended very early and for a couple of moves. In closed games this happens much later.
With this background pattern of undefended rooks and pawns and their transitions during the opening, UPS bookkeeping becomes much easier. If you know the rule, you just have to keep record of the exceptions.