The following ships are all static derelicts, for all gamestarts.
Maybe Egosoft will choose later to randomize their location, but unlikely as up to now (4.0 HF3) the locations are 100% static (checked through 5 game starts).
Update 10.01.19. - removed random derelicts and added a couple missing, from the fresh game start, found by tehnique from the BaconsTV/lordmuck youtube
Update 29.06.19. - removed main plot related reward position as the location is dynamic and depends on the sector where plot is initiated.
Update 02.04.21 - added new screenshots for location and more pointers to the actual location. Given coordinates are from the sector center, but just for the reference as they can't be seen in the game and it's hard to refer to sector center because of the sector dynamic scaling.
Update 15.04.23. - no changes for 6.0 and Kingdom End DLC - all 6 ownerless ships still present in the new 6.0/KE save.
1. S - Elite Vanguard (interceptor)


Location: Grand Exchange I (x=-332k, z=380k, y=24k)
422 k from Grand Exchange IV accelerator
362 k from Grand Exchange III accelerator
24k above the ecliptic
2. S - Perseus Vanguard (fighter)


Location: Silent Witness XII (x=56k, z=95k, y=0k)
234k from Silent Witness XI accelerator
183k from the SCA Pirate Base
3. S - Courier Vanguard (courier)


Location: Nopileus Fortune II (x=977k, z=1,175k, y=3k)
300k from the highway exit in the same direction. Picture submitted: @Submarine.
4. M - Osprey Vanguard (frigate)


Location: Company Regard (x=-147k, z=-185k, y=0k)
76.5k to Scale Plate Green gate
324.7k to Turquoise Sea gate
171.8k to highway
5. M - Drill Vanguard (miner)


Location: The Void (x=-274k, z=258k, y=-6k)
392k from Antigone Memorial gate
508k from Second Contact II Flashpoint gate
Note: Do not capture personally or come close to the ship personally (in sector). Ship is in the dangerous region and hull will quickly reach 0 and explode. After discovering the ship location remotely, send the ship with one rookie marine to capture, promote to captain and take it out of the dangerous region.
6. L - Odysseus Vanguard (destroyer)


Location: Faulty Logic VII (x=-165k, z=146k, y=0k)
313k to Faulty Logic I superhighway
266k to Atiya's Misfortune III gate
Note: recently (with 4.0 HF2) it is possible to capture personally (no need to use a marine), with full equipment.
Enjoy!