I'm glad that they brought it indoors for you but I agree with the football analogy above. They're making a mountain out of an ant hill here. Much to do about nothing. This isn't a crime scene. It was an accident and if you look at the footage found on that very drone they'll see you're clearly a hobbyist and didn't have ill intentions. (Even if you did have bad intentions, they aren't in a legal position to hold onto that evidence. They could turn it over to authorities, but not keep it on their own terms for any reason at all).
If you haven't gotten it back in a day or two of playing nice guy, I would start contacting the police. You didn't break the law, even if you had gone over 400'. Which, 400' is based off of where you took off, not their doorstep. Not to mention you can prove that you *intended* to stay under 400' because your displayed showed that you were under the ceiling and you have flight logs indicating the altitude. But again, even if you had gone over 500', you were just stupid but didn't break the law.
If I drop my cellphone in their lobby, would they presume I was taking unauthorized pics just because it's capable of something they don't particularly like and found it on their own property? Could they hold onto it even though they have acknowledged that they have it or would they be forced to turn it over?
I can't wait to hear how this ultimately turns out.