In some Ortona rules discussion with Perry, I learned this:
A
12.15 DETECTION: A unit forced back to its previous Location is not subject to attack (even by a minefield or FFE) during its brief period in the previously-concealed unit's Location, nor can it be the subject of a Snap Shot from the firer's hex while attempting to
enter the concealed unit's Location, but it is subject to Snap Shots/other First Fire attacks (see
C.5C) when being
returned to its previously occupied Location.
The defending unit can take a Snap Shot as the attacking unit is being returned to its previous location. In a building to building bump, that can be a better shot than shooting at the attacking unit in its previous location.