I lost a device. How do I protect my notes?

From any signed-in device, open Settings > Account > "Registered devices" and remove the lost one. The next time that device comes online it is signed out and its local data is wiped, within about a minute of its next use. It stays listed under "Recently removed" for 72 hours so you can confirm it is gone.

The honest limits: a device that never reconnects keeps whatever it already stored until it does, which is why the app lock (PIN or biometric) plus your OS screen lock matter on anything portable. And removal is not a key change - anyone who actually holds your 12 words can sign in again regardless. If you suspect the phrase itself leaked, follow the phrase leak playbook instead.