These are ordinary mission-generated planetary outposts/buildings - while you may not have taken a mission directly, if you were more than 15 minutes in your ship or SRV, you could have been given a "hidden mission" by the game to generate one of these for you - usually will have an interesting data point which you can scan for money.
That's not exactly what happened. In SRV, see a contact 17ls away. Lock it in. Return to ship. Take off. Contact disappears but still locked in. Fly to another planet completely and land at contact point eventually. When approaching, the contact is CLOAKED. The location alters multiple times and you have to weave in different directions. Finally the contact (not visible as contact on Contacts at all) settles with a final destination and the base materialises.