You know when your U2F device has been stolen because it's not in your possession anymore. The hardware is meant to be at least tamper-evident, if not tamper-resistant, so an attacker can't just steal the internal secret and put the device back where they found it.
Bytes in a password manager are hard to steal, but if you do steal them, the legitimate owner won't necessarily ever know.
Bytes in a password manager are hard to steal, but if you do steal them, the legitimate owner won't necessarily ever know.