Dark night, running silent, scope down, 100ft depth = passively detected by escort?

[Hurin]

Hi All,

I'm running FotRU and I'm nice and lined up on a task force, aiming for a Heavy Cruiser. . . but unfortunately, there's a destroyer coming up its side and it's going to pass 2-300 yards behind me.

No problem, I think, I'll lower the scope and wait for him to pass. I lower scope, I'm already running silent, all stopped, and it's a very dark night with a bit of fog (though seas are calm). I'm in a Tambor class so I'm at 60 feet. I set depth to 100 feet and wait. . . and right around 300 yards, I'm detected and the whole task force goes nuts.

I'm not complaining, but I'm just wondering by what game mechanic I'm being detected? Just a lucky passive sonar operator that close to me?

Thanks for any advice.