As for 'why not' to use the malfunctions/sabotage option, I can only hazard a guess: that the random sensor breakage method used, may not be able to avoid periodically breaking the visual sensor. (U-boat crew on watch)
Regarding smashing into the vessel, maybe you somehow slipped between its sensor arc sweep. I dunno. I've had WTH moments on occasion without clear cause or consistency too, but unless they show a pattern, I just try not to fixate on the matter.
Cheers, I'm going to go look at something shiny now.
