Both Thargoid structures and Guardian structures are mostly concentrated in certain regions with a few hundred lightyears diameter and not randomly scattered throughout the galaxy. So if you look in the right region, it is possible, but quiet hard since these regions have been explored quiet intensely as they are not that far from the bubble (within 1000ly).
I wouldn't be surprised if there are still some undetected guardian sites or even beacons out there around the region where the new ferry shuttle is traveling to, I discovered some myself at the time the new sites and beacons where added.
As for Thargoid structures, I don't really know how likely it is. They are located around the Pleiades, and IIRC each one pointed to other structures, so they were all found relative quickly.
As for where they are located, you can use this site:
map.canonn.tech