In every SH3 convoy there is a "lead ship" - it will not necessarily be the ship out in front... when a circle appears it means a submarine is the lead ship in the convoy, so you will want to be on the look out for one of two things.
1. an enemy submarine within the convoy on the surface serving as an escort.
2. Other friendly U-boats attacking the convoy.
* if it is #2 you will likely arrive after the fight is over, be on the lookout for damaged freighters
