Why the "Sign-in error" notification appear on Xiaomi device? Since yesterday, I've been getting this error message and I have no idea what application it is. I checked all of my installed apps and can't see it.

Can you help me identify this icon?
You must be using Xiaomi device with stock ROM. When you first switched on device, you created an online Mi account like Google account. Now this device is attached to that Mi account to avoid misuse in case of phone theft. This phenomenon is similar to FRP implemented by other OEMs too.
Shown number 178... is your Mi account ID. This error is appearing because device is unable to verify your identity online due to no internet access or something like that. Go to Mi Account in Settings and login again.

Be careful! If you are unable to login, you won't be able to access device on reboot.

See this to know a little bit about how FRP works.