1.Hi nik112 if you are on the Surface or at periscope depth theres an imaginory detection box around your Uboat at periscope depth that box will grow larger or smaller depending on the speed you are doing and the noise your engines make in your Uboat sound carries under water also when you are on the surface doing 7knots heading to your Patrol Grid look at your wake behind you from 500-1000ft above you that wake is like a finger pointing where you are to any patroling Aircraft
if you were a chicken you would'nt want to walk past a starving fox play to your strenghts be silent and deadly aways stealth is your best weapon
and when you have attacked a ship or Convoy in one Grid move to the next Grid dont hang around that area.
2.Also depends on the crew class of the escort/escorts and the year of the war as better escorts arrive in theatre with radar they see you before you see them.
3.When playing the GWX Campaign or LSH Campaign i only surface at night to recharge my Uboats batteries at dawn i dive to Periscope depth and cruise at 2knots when im near or in my Patrol Grid.
4. Also when im near to my Patrol Grid say Rockall banks area AM01,17,18,19,41 i only use time compresion 125% then i get fair warning of air patrols.
Hope this helps you some nik112 Salute Sir check this World at war Documentary on Uboats and Wolfpacks and Allied tactics and new equipment to counter the Uboat threat as the war progressed its a good watch enjoy
https://www.dailymotion.com/video/x3p7bjc