Yes - that sort of pop up on the main map may be valid in a counter insugency type game though (insurgent appears out of a sewer, or it represents a "civilian" pulling out a grenade or whatever
However, on an open battlefield, arriving reinforcements should probably only appear right at the map edge they arrive on (within 1-2 hexes maybe), and not "teleport" into the main playing area .
Any of your troops who were hugging that edge and then suddenly found enemy troops in thier midst/flank/rear can be assumed to have been outflanked/and or ambushed by the enemy reinforcements arriving unexpectedly. So you will have to pay some attention to flank security on the map edges, and cannot then assume it is "dead ground" beyond the grey hexes
!
Cheers
Andy