Beta Lanki Patro mission in the Tau sector (New Roumulus). The mission where you defeat Tholians to beam up releics. After the last relic is beam, sometimes, you get a message from your crew sayng that a strange ship is approaching (assuming it's Iconian). You can see this ship in the distance (purple-ish). When this happens, you'll see this ship for a few minutes, then it warps away. You cannot target it. Afterward (and I think this is the bug) you never get the dialog box asking if you want to depart the system. You have to manually do that with the icon upper left of your map.