Your Nest account is usually the main entrance point for hackers. It would be a good idea to change your account password and add two step verification to help prevent unauthorised logins. Two factor verification is different depending on the Nest app and Google Home app. It looks like you tagged the Nest app, so this information should help: https://support.google.com/googlenest/answer/9295081
Often the reason of unknown account access is password breaches of other services as well as password reuse and credential stuffing techniques. Setting a unique password that's not the same as any other account along with additional verification like 2FA prevents this type of attack.
Migrating to a Google account if you havent already does offer more security options for additional login verification, but I know many still prefer the Nest app or have not migrated or linked their Google accounts. If you have migrated you can use many of Google's multifactor authentication options, like passkey, one tap notifications and more, meaning you always need two parts to login, ensuring it is really you.
