This might be quite stupid as I don't use map contact updates and my experience with convoys (in SH4) has been collected from a periscope/hydrophone point of view.
However, I think there is a gap between the escorts that mostly stay around the side and the escort that stays ahead of the convoy - if you wait to the side of the convoy I often manage to slip closer after the first destroyer is past and before the second shows up. Needless to say this also goes wrong sometimes and they pick me up.
Hey let me just illustrate what I mean with an ugly picture:
