Slitherine

Why would a transport bump into and reveal a sub, but not suffer any strength loss?
Poor man's sonar?

I understand only one unit may occupy a given location so the game needs to halt the transport.
But in this case the transport should suffer a regular sub-attack.

Edit: And the sub's exact location should not be revealed forcing any destroyers to actively hunt for it.

+1

I get the objection about the game forcing the transport to stop - a programming quirk that probably is a lot harder to fix than it seems - but I don't see why the transport would necessarily suffer damage as a consequence of encountering a submerged sub. Unless the sub chooses to attack, the supposition would be that the transport merely passes over its silent foe, oblivious and undisturbed. But yes, the proper thing would be to NOT reveal the location of the enemy sub even if the transport moves right over it.

I have no problem with the transport or any naval unit moving over it.
That would be my preferred solution leaving it up to the sub to choose when to attack.
But you would still need to find a good solution for ships that are plotted to end their movement in the sub's location.
Tricky.

To be clear: Subs are NOT revealed when an enemy moves through their hex, only when the enemy tries to STOP on the hex. There is no way 2 (naval) units can share a hex, so there isn't really any other solution to it. You can assume it is the surface ship's crew spotting the subs periscope

As for an auto-attack: If they auto attack an "ambushed" transport, then why not carriers? battleships? low strength destroyers? It gets very hard to get a consistant and clear mechanic. As it is the sub can decide what to do in its next turn. In fact, if it auto-attacks, it would lose torpedoes which you may have wanted to use against another target.