Ah, that'd be the Thermobaric Tank, from the end of the "Protect the Thermobaric Tank" story mission. Sadly, you cannot (to my knowledge) ever find/drive one again. At least I have never seen one. Yeah, it sucks, I know.
Which brings me to a question of my own. Now, I realize PWizard and Caliber are effectively (okay, "effectively" nothing, they've said they're done) done with the Trainer(s) for this game. That said, does anyone know of some way to spawn vehicles in front of you a la Grand Theft Auto Insert # Here? I dunno, using CE or AM or something? Alternatively, if PWizard/Caliber WERE to put said feature into a later Trainer, however unlikely that possibility may be, that would INDEED be AWESOME. And greatly appreciated, as I know I'm not the first person to suggest this.