If a player runs a convoy immediately after accepting the mission then it will take a couple of seconds for the convoy mission, the "Attack" *on, to appear on the plunderer's screen. This is the scenario you are talking about. But if a player runs a convoy after at least 15 seconds from accepting the mission then the convoy mission will immediately appear on the plunderer's screen.

That simply means that the player being plundered was waiting at their group base for a couple of seconds before setting off.