High orbit mining
While holding on space high orbits Scourges will release a gas cloud that cloaks them from common space scanners. This allows them to stay there invisible to galaxy radars, so the enemy won’t see Scourges among the fleet units while attacking the high orbit or viewing the galaxy.
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.