There are differently-colored lines for supply lines (over land) and sea lanes (over water).
Sea lanes are more valuable because they help you do Sail / Charter Boat actions, whereas supply lines do not (boats can't go over land).
I thought the fact that there was only one city remotely close to the recon box made it reasonably clear where you were supposed to be in order to do the recon.
If you think that doesn't indicate that, then why limit it to yellow cities? Why not allow the recon from anywhere in the world?
I assumed that something else would tell us what to do to be able to recon in a future month. When 4 out of your 5 initial recons have a solid, straight, black line, that doesn't exist anywhere else on the map, it's confusing when the 5th doesn't have that line.