"Even without the stealth optimization planned for the SP project, Elofsson added, it can be said that the signature reductions achieved on the Visby class have been impressive. "The crew of Helsingborg experienced this during their transit through the English Channel when they were contacted by the UK radar surveillance centre at Dover who picked up the ship`s AIS transponder signal, but did not see the corvette on radar despite the fact that Helsingborg did have some radar reflectors deployed to artificially enhance its radar cross-section." In order not to compromise safety, the ship decided to put up additional radar reflectors, Elofsson claimed, while Dover issued a radio warning to all ships in the area about the presence of the stealth ship."
http://en.wikipedia.org/wiki/Visby_class_corvette