I have managed to spawn a Sentinel by

• being on a rocket rampage
• being where the screenshot shows
• having a Taxi
• and making and spamming very short replays

The Sentinel just appeared out of thin air. It has some interesting features:

• You can morph it into other cars via a replay. It only works in certain locations in the game, and the car that replaces the Sentinel is dependent of the location.
• It can't take physical damage and the damage on the hood can't be repaired.
• The game doesn't seem to recognize it as a car. Save file editors can't see it.

Vid of the morphing:

Vid of the circumstances of the spawning:

Link to the save file with this car: https://drive.google.com/file/d/0B38XMNjp-HeiNW9HUlRRSERfVm8/view?usp=drivesdk

Is this useful? No. Is this interesting? Yes. Can this be reproduced? Probably, I haven't managed to do it.

Apparently it's the car that you get for the Thieves mission:

<blacklev>: powdinet and other 100% runners: when does the game say that "return when you aren't so busy" when you are trying to export a car? because the ghost stallion can't be exported because the game says that, could it be a mission vehicle? <blacklev>: i mean ghost sentinel <Powdinet>: it says that if you are in a mission vehicle or are in a mission with no set vehicle afaik <blacklev>: which mission gives you a sentinel? one of the dog food factory ones? <ultimaomega07>: thieves <blacklev>: does it have a set color? <ultimaomega07>: looks like it <blacklev>: i guess it's blue <blacklev>: then i'm pretty sure that is the car that spawned for me <ultimaomega07>: dark-ish blue <blacklev>: # that <ultimaomega07>: ye <blacklev>: can you visually damage that car in the thieves mission? <ultimaomega07>: yeah <ultimaomega07>: you can take off the door and shit <blacklev>: if you store it in a garage, can you still visually damage the car? <ultimaomega07>: never tried <Gael>: blacklev: stap breaking gta3, just stap <blacklev>: it seems pretty obvious that it is the same car i got <Gael>: poor game <blacklev>: but how the hell did that happen

So, what happens if you start The Thieves after spawning this thing?

Unfortunately, Sentinel from The Thieves doesn't have a set color. It is worth noting that the car is saved regardless of whether it is in garage or not. Also running 'The Thieves' from save works fine, and finishing it doesn't affect existence of this Sentinel. As any 'marked' car (for example, glitched by replay) it doesn't disappear, unless it is destroyed, even in cutscenes.

Also it may somehow be related to Gone Fishing mission, because it spawns Sentinel somewhere close to this location (closer to fixed Patriot spawn).