If, and it's a big IF, the US is onto something in the Indian Ocean, then a whole lot of questions suddenly pop up, don't they?
Like how come RR and Boeing apparently have no data supporting the theory?
Like why the heck they've wasted 5 days searching an area where they seem to think the aircraft didn't go down?
Like who is actually directing the SAR ops?
And many more...
When a radio transmission is attempted but fails because of no hand shake response from the ground station, it will retry at a set interval. If the a/c is out of range from ground receiver then other sensitive (surveillance) radio equipment might passively detect these pings without responding. The signal can then be decoded given the appropriate signals analysis equipment.
Think of your cell phone. When out of range it sends a ping periodically seeking a handshake back from the tower. A radio receiver can detect that ping passively without any response and the receiver could even decode it if they have the right equipment. This is one explanation of how the signal was detected yet not relayed to RR or Boeing. To clarify the ping is only a short high power signal giving some very basic ID and ready to send data. It is not the full data transmission. If you have more than one receiver it is possible to triangulate the transmit location.
This technology in WW2 gave British fighter command an early warning of the German bombers coming over for raids. Needless to say signals analysis has come a long way since then.