I keep firearms in my truck, but I make sure its locked, and on the off chance I forgot to lock it, I make sure the guns are still there. I also make sure nothing is visible that could convey any value to anyone, reducing the chance further that someone might break in.
To the casual observer, it could have been pretty obvious you transfered the pistol from one vehicle to another, then left the vehicle unlocked and unattended. To the casual observer who is also a thief, you just left the situation wide open for a payday. I don't buy the story that he wanted to buy it. If that were the case, you'd think he would have approached you when you were around, or your wife who was right there.
2 failures here after reading the thread: yours, for being a bonehead and leaving the gun unattended, and announcing it to anyone within earshot. The other was hours wife's lack of situational awareness. Not only did she miss the presence of the thief, but what if the theft of the gun hadn't been his only intention. She missed the presence of a man with a loaded, stolen firearm.
You've learned your lesson, did she?