It *sounds* like an exploration mission....so try the exploration clusters, I'm reasonably certain it's not a 'proper' mission.
Agreed. It's probably a mission the OP picked up, but didn't complete.
OP -- go into whatever star clusters you've visited before, and see if a tab appears on the right side of your screen. If it does, that's where that mission is located. You can then click the tab to go to that system and complete the mission.