I might be mistaken here..
Detection ranges are mainly determined by the surface area you present to your target - IOW, Side-On (bearing 90 or 270) presents a larger 'visual' or asdic area than facing directly (bearing 0 or 180).
From here there is a reduction in % with regard to range, weather, light conditions and maybe other factors like
blind crew members. Over a certain threshold, and you're (or they're) detected.
I think there is also a time limit for detection and loss of contact.
The scenario is not perfect by no means, and can be darn right stupid. This is why you have to play Captain and crew at the same time.