An Asgard invisibility device is a small hand-held cloaking device used by the Asgard. When the device is applied to the body of an individual and then activated, the person becomes invisible. This device was used by the people of PX3-595, an Asgard protected planet, to hide from predators. It is unlikely that this device blocks people from sensors, although it does block scent and sound, however, because the Asgard would want to limit their technology given to the planet to the minimum necessary.
In 2000, SG-9 attempted to negotiate for the device, but was unable to obtain one. Harry Maybourne later sent a rogue team temporarily led by Jack O'Neill to steal the device, which was later brought back to Earth by Colonel Robert Makepeace. Presumably, the device was returned to the Asgard along with any other stolen technologies. (SG1: "Shades of Grey")