The fact that your key fob was 500-feet away is not evidence that it doesn't have the capability of communicating with the car. Unfortunately BMW does not shield their electronics and you'll not the FCC warning indicating your car is susceptible to radio interference.

A few questions:
1) Do you live within sight of high tension power lines?
2) Do you live within sight of any radio towers?
3) How are is the nearest cell phone tower from your home?
4) Where is your electrical circuit panel box, in your home, compared to where you place your keys or park your car at night?
5) Was there any lightning the night of the storm?
6) Do you own a modern microwave?
7) Do you use a modern cordless phone? If so, what GHZ?
8) Do you have wireless video game controllers?
9) Does anyone else close to you own a BMW?
10) Have you ever washed your key fob by accident, in the pocket of your jeans?
11) Do you own an iPhone?
12) Do you own any high powered stereo equipment or computer equipment?

The questions above, relate to the fact most modern electronics are "dirty" in that they release rouge radio transmissions that can disrupt signals, mimic signals, and cause unusual malfunctions. Very few consumer electronics are shielded from that interference.

If there were shielded, you'd know about it, because they would have cost more. It's possible your BMW was subjected to interference from any number of things, causing it to malfunction. Even a military jet flying overhead could have activated an electronic countermeasure, by accident, or could have been testing such a system if you live within a few miles of an airport (even a commercial airport). The point is, this is the unfortunate result of what can take place when electronics are installed into any device whatsoever.

Of note, each time I take my iPhone into my wife's brand new honda civic, the radio will start to hum at audible levels, even though the system is completely off. It's annoying as you can imagine. The only thing that will make it stop is turning the iphone completely off.

It's quite impossible to say what took place, but if you'd bet your life on the fact that you did not accidentally hit the key in your pocket or within your house (or your cat didn't jump on it), look at the list above and see if any of the items apply to your situation.

The fact that you're within a concrete home, 500 feet from your car, is unremarkable, again, as many items can act as antennas and can extend the range of your key fob by hundreds, if not thousands of feet.

Good luck.