A ship must have its transponder turned off to attempt stealth (otherwise, the ship automatically broadcasts its identity, thus defeating the purpose). A ship must also have its shields down and not be using active sensors in order to attempt to hide.
The DC for using active scanning to detect a (hidden) passive scanning, transponderless ship is calculated using its Target Lock AC. A ship attempting to detect a hidden vessel using only passive scans is assumed to roll a 10 on this check. (So passive scanning to find hidden ship is 10 + Computer mod and active scan would require an actual roll.)
A ship rigged for "Silent running," that is with all nonessential systems powered down (as if the power core were disabled, and running on batteries), receives a +5 to its TL for the purpose of detection. (So turn everything off including the thrusters and basically just float in space)