Its a stealth drone. Its very hard to detect with a radar.
More than likely wasn't radar that detected it.. The Russians recently introduced some new ECM equipment in Iran.. When the control / vehicle distance is extended, it begins to get easier to pick up the transmissions back and forth, if indeed it was hacked and landed under its own power it could have been something alone these lines, also encryption more than likely isn't nearly as strong during certain parts of flight paths, especially takeoff and landing sequences that are time critical, it's hard to say when it was actually pirated. One thing is quite probable, however, which is it wasn't Iranian proprietary equipment that let them do this.
-Edit- Not sure on the fact here, but thought I heard it was supposed to fly over Afghanistan then went off course, this would support the theory that it was pirated during the takeoff and ascent portion of its flight path which would probably have the weakest form of signal encryption.