Very simple:
How can an escort detect you from 800/900m when you are submerged and all stopped with silent running on, just waiting?
No matter if at Periscope depth with no raised scope, or even 50m depth?
Do they have Japanese kind of medium aboard?
Calm sea or heavy sea without difference?
Very very rarely i can remain undetected, so i suspect it has to do with the escort skill (noob/veteran/elite) not to say elite can see you wherever you are or staying...
I’m not sure in reality Japanese escorts were that good at detecting sub all stopped from hundreds meters away.
They don’t in SH3/SH5, but they do in SH4...