While in high orbits in airless space emit gas and are perceived by galactic scanners as stardust. This results in the invisibility of the scourges seen as stardust when defending (recycling) in high orbit to other players in the galaxy and when flying into that high orbit. The scourges will be absent from the fleet composition in the Fleet panel.
How to counter:
1) Reconnaissance-in-force. You can send a small fleet to the high orbit that will show if there are any Scourges.
2) Sensor Phalanx can show you the full composition of any fleet at high orbit.
3) With a hired Telepathist from a hidden satellite, send the fleet into high orbit and in the Overview menu look at "See Object", if the object visibility has become available to other players, then there are scourges in high orbit.