If you can get away from escorts after sinking or crippling a merchant, the best way to hide is Silent running and 1kt. Depth is up to you but 60 meters is good. In my last 4 patrols I've always slipped away undetected.... and that with deckgun onboard.
Tho in 1943 I don't use the deckgun as much as I did in the earlier years. But then again, you can still run across convoys occasionally with all merchants unarmed. It's the escorts and the number of them in a convoy that's the problem, lately I've been seeing 4 up to and 6 even.