The detector shield was a device that Avon worked on one for Liberator. He had apparently been working on for a long time, finalising whilst it Blake was isolated himself in the wake of Gan's death. (TV: Trial)
The shield kept Liberator off everything but the Federation's close-range visual scanners.
Following a test run, it enabled Blake to make a surprise attack on Servalan's HQ (TV: Trial). It was in operation whilst in orbit around Fosforon (TV: Killer) but during a attack from Federation shortly prior travelling to Exbar, it had broken down with some required parts unavailable. (TV: Hostage)
It seemed to have been repaired by the time the Liberator journeyed to Star One, with Jenna mentioning it was activated when Travis's ship arrived. (TV: Star One)
The Federation had also had developed such a device, using it to launch a surprise mass attack on Liberator. This seemed to annoy Avon, who said he had been planning to sell them the details of his own version. (TV: Hostage).
The device was referred to as "ghost mode" by Orac and "stealth mode" by Cally when it was activated during Liberator's visit to Erebus. (AUDIO: Erebus)