Since the FBI isn't saying what sort of evidence they have that the girls are alive, it is impossible for me to evaluate the statement. Are they going on getting sighting reports from other places? If so, well, that is discouraging to me because people are still reporting seeing Elvis.
As for the smell... Iowa is a largely agricultural state. If the perp killed both girls and drove 15 miles, then dumped the bodies in a corn field, there would not be anyone around to smell the bodies.
Even if someone did smell the bodies, Iowa is full of whitetail deer and they do occasionally lie down and die rather than get shot or hit by car. What is truly amazing is how fast the scavengers reduce a dead deer down to little more than bones and scraps of hide (less than a week).
If the bodies were dumped in a corn field, it's unlikely that birds would be spotted, since most birds feel uneasy flying into 5 foot tall cover (should be 7 foot but the drought has taken a toll).
If the girls' bodies are in a corn or soybean field, this fall will reveal them.