According to this report, the Bolivian plane had to land because of a faulty fuel gauge:
Control tower: Do you need any assistance?In addition, France and Spain both deny forcing the plane to reroute, though Spain says it denied entry until they found out the Bolivian President was on board.
Pilot: Not at this moment. We need to land because we cannot get a correct indication of the fuel indication so as a precaution we need to land.
The plane was searched. But, if anyone was paying attention, and I bet the Spying US Government was, Snowden could not have been on the plane. The airport the Bolivian plane took off from was 27 miles, on the other side of Moscow, from the airport where Snowden is.