Indeed, this is where storage gets way too complicated to make it a cheap thus affordable and reliable solution.
You either go for the cheap and easy way to deal with, like placing a tracker like an airbag in it, but which under 10 feet of concrete and rebar won't be detectable at all, Bluetooth doesn't penetrate that deep, or you go for a more complex and powerful solution which will need a third-party service, a huge battery that will increase the size, which might solve or add another problem on top of an existing one.
With a small box, it's highly possible it will not get noticed among the ruble just get thrown with the trash in a truck and dumped before you're even allowed to reach the scene of the disaster, with a huge box, there is the problem of it getting stolen as some might mistake it for safebox before you have the chance of getting it back. Plus if you make this big and durable, you have to hide it also pretty well in your house in normal times for the same reason, thieves getting in and thinking it's a safe box full of money, which actually it is
Yeah, easier said than done.
It was hard for me to find a place to store a pair of keys in case both me and my wife lost ours, (obviously, the inside of the house was a no), and with a hardware wallet, you must exclude a person keeping those, so, how many people have 2- or 3 properties with land so they can bury a case or a box far away from prying eyes?
The strange thing is that I feel safe and not even think about developing solutions for such days. I do not know, does everyone feel that?
If you're living above the third floor or even better you don't have tsunamis at all in your region there is not really a matter of concern. But you could at least laminate it to avoid normal degradation or accidents like your neighbor flooding your apartment.