Precious things car keys nowadays so they're worth investing in at least your mobile phone number on them as I'd certainly ring the number if I found a set so you could arrange to pick them up from me 
https://www.ebay.com.au/itm/352715995674
Such a simple but effective idea
Not a fan of the idea, too scared of it gettin wacked by flying rocks or losing that one ...
That's my concern too!
Hiding / securing / secreting a key under or on your vehicle will almost certainly compromise your claim if your vehicle is ever stolen.
Just saying!!
Definitely a good point to consider....
