I just came into a system and saw this on the map.
On closer inspection they turned out to be ore markers. I have never scanned or mined anything this game nor had I been to that part of the system.
Ore markers appear when anything, player or NPC, successfully mines asteroids.
I have sent mining-enabled ships from my own mods to mine and markers appear on the map as they mine while my playership is busy doing something else elsewhere.
From the looks of the posted image, my guess is something, either Outlaw miner or Ferian warrior, shot those asteroids when they fought Rogue Fleet, Xenophobes, or other faction aligned with Destructive Chaos while you were minding your own business.
Download and Play in 1.9 beta 1... Drake Technologies (Alpha): More hardware for combat in parts 1 and 2! Star Castle Arcade: Play a classic arcade game adventure, with or without more features (like powerups)! Playership Drones: Buy or restore exotic ships to command!
Other playable mods from 1.8 and 1.7, waiting to be updated... Godmode v3 (WIP): Dev/cheat tool compatible with D&O parts 1 or 2.