But what about past activity? Say that a person had put $100 in BTC into an unencrypted wallet for the last 10 months, and he then encrypts the wallet (Bitcoin CORE).
Someone finds an old unencrypted wallet.
How is it possible that it would not have valid content that was retrievable by the finder?
If someone finds an old unencrypted wallet and your old addresses still have coins in it, they can spend it. You have to send it to a new address in your new backup to deter this possibility.
This is what I thought. The popup message which comes up is then quite confusing.