A stopped boat is a dead boat is my motto.
Actually, because of the way sonar works, there is a "cone" of detection (depending on your profile) from the surface to depth. Being below or above this cone decreases detection (again depending on how close you are to the hunting enemy.)
In addition, if what YOU consider to be a "safe" depth is above the layer, your illusion of your relative safety will be sorely tried. A warship task force usually has very aggressive escorts. You should never under estimate them.
Being at a dead stop is RARELY a good idea in this game. Continuous movement of one kind or another is best. Even in the situation you describe, if the enemy gets a momentary contact, sweeps back again to check and you're not there anymore, then you get away. When you're not moving, the enemy gets to listen or ping at that spot all they want and will eventually snag you. I also suspect you had a poor aspect angle on the closest destroyer making your detection higher.
There are other noise sources on your boat besides engines which you can control to further increase your stealth. The only time I ever come to a complete stop is where there is no enemy around and I'm waiting for the right timing to make my move (usually a harbor incursion.)
-Pv-
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