What luck! You found $1M in a suitcase while hiking in the wilderness. Because you are a law abiding citizen, you diligently report all your found money to the IRS. No one, including yourself, has any way to prove that you did or did not cause this money to be where it was or that you did or did not know it would be there.
Are you within the law?
Being more serious, is unexplained money in and of itself illegal if you report all your income to the IRS? If an explanation is required, are "I found it", "anonymous gift", or "I decline to explain" a sufficient explanation?
On face, it sounds draconian and absurd to outlaw finding money. On the other hand, it's likely that someone who regularly "finds" large amounts of money is an inept money launderer and engages in other illegal activities. It wouldn't be too surprising if simply having large amounts of unexplained money in and of itself was illegal.
Does the situation change if instead you find a bag of diamonds (or any other asset) while on your hike? What if this happens on a regular basis?
I'm interested in US law.