There is indeed a stock bug in vanilla where the snorkel has the same visual signature to the AI as does the entire boat. I
believe the GWX team fixed this, but am not sure. If so you should be safe snorting at night before c. mid '44 (when millimetric radar started seeing widespread installation on Allied maritime patrol aircraft).
[Great, now I'm saddled with the dreaded "Medic" avatar...
]