Well, however long it took to get there, might have been local storms prior to your arrival. Also, the game gets really squirrelly if a "ship" is set in too shallow of water, of which, the Target assets are invisible ships (to the player's eye anyway), so it might be that they self-destructed in the shallow water. Plus, don't forget about the "drift" factor that is alive and well in the game. Besides other unknown "requirements" of the game, the Targets should have a Speed=0, a waypoint set at least 1000m away, a "Head to waypoint" set, and a GameEntryDate and Time set to a time near mission time. You could also experiment between "EvolveFromEntryDate=" of "true" (the asset is placed where the game calculates it would be when it "spawns" from you getting close enough) and "falses" (it spawns where set when you get close enough). Triggering the asset into existence is also an option, though does complicate things needlessly sometimes...
