The Airtag thing did occur to me, since they beep when you look for them (right?) Otherwise "it's in the house" won't help much when you know that anyway. An SDR solution could presumably help locate it based on signal strength. Also we're not an Apple household.

One thing against a locator gadget is that these (Honda) keyfobs have the keyring on the detachable part (with the physical emergency key). Frequently we detach that and just take the keyfob, in a securely zipped pocket, when going, say, grocery shopping. That makes the fob easier to misplace, but the locator thing would still be on the keychain.