Here's your help.
Tell no one.
You might get someone helping themselves. Which while disappointing , is better than you being eliminated physically in the process.
Rent or buy a two box.
Practice with it off your declared treasure site.
Be danged sure you have permission on the recovery site.
Know the legal implications of recovering such item(s) as you anticipate.
Know who is watching. And before you say no one is , look up! Then out a mile full spectrum.
How you going to hide the contrast of fill removed while digging?
Having permission to be on site and a tent with no floor material comes to mind. No guarantee that would work. Being watched recover though is not going to bode well if you are not legally and financially prepared.
Expect your secret to not be secret.
It already isn't!