I mean if you're outside in the open and there's a ship in the same square you are in and it has more sensors than your stealth skill then you're detected right? So what happens if you're one or two or even three squares away from the ship, but the ship has 5/8 sensors and you have lets say 2 or 3 in stealth, how does this work? do you get detected?

Your sensor value deteriorates with distance. The Scanning rules have all the info.

Wow, so stealth is actually worth going for, thanks :)