The instruments on board Venus Express had been developed as back-ups for either Esa's Mars Express spacecraft or Esa's comet-chaser mission, Rosetta, due for launch in January.
Furthermore, because the effect is present in data from five vehicles Galileo, the Cassini mission to Saturn, the MESSENGER craft sent to Mercury, and the NEAR and Rosetta missions to study asteroids and comets the team thinks it is unlikely to be caused by the spacecraft themselves.