Some Nest users have been having problems of late with people gaining access to their accounts and spying on them, using their own security cameras, according to reports. The reason for this in some instances is that the Nest users chose passwords that were ineffective or left at default. Google has sent out emails that were very vague warning users that someone else having their credentials could cause the issues that some other Nest users have reported. That email tried to get users to choose better passwords, but most people didn't bother and chose to ignore the Nest team's guidance.
Nest's parent company, Google, is also now taking it a step further and is proactively locking people out of their accounts, if Nest believes their account has been compromised. Once locked out of an account, the user will be unable to use the Nest app and receive notifications from their cameras or any other Nest device, as would be any unauthorized user. However, if you have a Nest Protect smoke alarm, its integrated alarms will still work.