Sneaking units

Hi there.

In Dom2 we can see the "sneak bonus" of all stealthy units. Most, including scouts, have 0. Spies and assassins often have +5. Some commanders even have +30.
But do somebody knows what this means exactly ?
Is this a % bonus ? But what is the base chance to be caught ?
I also read in the manual (p. 11, nr 4.8) that local defense (even 10+) is less efficient than real patrols -- is local defense still useful then, for the price ?

TIA,
Sunray