Important item in Keychain seems to have disappeared (after years)

I had the following code in a program that I used to encrypt some important files. I haven't run it in a few years. It used to work, and now it seems the password is mysteriously gone from my Keychain! The return value is now errSecItemNotFound.

I'm upset with myself for not backing up the key/password somewhere else. Is there anywhere this could be hiding? Did Apple move it somewhere? I know they created this "Passwords" app in recent years, but I don't see anything in there with the "account" string I used. I run the app from Xcode, so maybe it is in the "container" data somewhere? I do see keychain files under ~/Library.

Maybe there is a way to look through old Time Machine backups. Ug. So stressful.

Just looking for pointers on where the data might be, and why it might have disappeared. Unfortunately it was not a "guessable" password, it was a generated 256 bit key, base64 encoded. Perhaps I could crack that with brute force if I'm determined enough...

public static func queryGenericPasswordAsString(account: String) throws -> String {
    let query: [String: Any] = [kSecClass as String: kSecClassGenericPassword,
                                kSecMatchLimit as String: kSecMatchLimitOne,
                                kSecAttrAccount as String: account,
                                kSecReturnAttributes as String: true,
                                kSecReturnData as String: true]
    var item: CFTypeRef?
    let status = SecItemCopyMatching(query as CFDictionary, &item)
    guard status != errSecItemNotFound else { throw KeychainError.noPassword }
    ...    
}