You get around to doing that yet? Things have the tendency to get lost in the weirdest locations, I wouldn't spend a penny until I overturned every rock that hasn't budged for years and especially look in every overgrown patch. Pretty sure I've found at least one thing in that way. Don't put it off as the rarest of possibilities; it probably won't be there but that's where I'd be checking if I hadn't yet.
edit: honestly, if not there, I'd probably look everywhere in the nearby area, areas I specifically never went near, perhaps the entire yard and trying to look in nearby yards. Why? Animals. Animals can be dicks. Seriously, one could have picked up the keys and dropped it elsewhere. If it was one of the craftier crows, it may have kept it. lol
I'll check them now, been on the phone 90 minutes with locksmiths and AAA. Only animals here are cats and they wouldn't touch keys.