I imagine that theft wasn't the motive at all in this case. I think someone was trying to teach this guy a lesson. Perhaps a neighbor knew how he stored his guns? A friend who wasn't happy with it? A person walking by who saw it? Who knows...
But, that behavior is far too thought out for your average thief, and doesn't smell of a criminal motivation. Also, the news is sometimes inaccurate in their reporting. From the way the article is written it sounds like the vehicle was parked outside. In Colorado that incident would not be a "burglary", though perhaps it is in Florida.
Still, having investigated hundreds of burglaries, and theft from motor vehicle cases, I'm going to wager that this was not a criminal in the sense that the article is implying.
By the way, I agree that this guy was stupid: leaving your gun in an unlocked vehicle that is accessible to the public. Really?
The criminal who committed the felony is comparably stupid.
There's really no felony on the basis of what was described (based on the laws here in CO). If a person walked by, picked the gun up out of the unlocked vehicle, then placed it on this guy's porch with a note to him, there is no felony. Perhaps a misdemeanor theft for the ammo, but that's about it. If the person had stolen the gun, and if the gun was worth more than $1,000, a felony could apply. Similarly, if the person had actually entered the vehicle, then a Criminal Trespass could apply. Otherwise, not much going on here. Only way to really turn this into a burglary was if the car was parked in the "victim's" garage at the time that the item was removed from the vehicle.