I few things I would add to this.
Your metal parts may be in fine condition, but if the stock is ruined, you have a problem.
Always consider “biologicals”, like ants, termites, and larger animals.
If someone is using a metal detector, it is likely you are already a suspect, so all security measures should be in effect.
Deception pays. If they dig up 6’ of dirt over a hundred square foot area and find just enough non-incriminating evidence to do that, they will have likely lost their taste for digging.
Planned disappointment is the best. Imagine digging down 6’, because the metal detectors say “big metal” is down there. Then finding a large, non-functional safe bought cheap, with its door welded shut. Then, after they painstakingly open it, they discover only compost, 2” of nasty black water, and a few pounds of very fetid pork grease. Very demotivational.
If you have a well, burying next to the casing would also disguise almost anything. A 6” well casing puts out a huge signal to a metal detector. I work for a water company, and could probably bury bazookas and bangalore torpedoes next the well casings.