Is it possible that someone is accessing my iCloud account?

When same apple id and password on the same network is signed in between your iPhone and other devices and you have shared all account credentials it can be verified using the steps in the article Check your Apple ID device list to find where you're signed in - Apple Support

The iPhone can be removed .

Read the section - Remove a device from the list

You can also follow the articles If you think your Apple ID has been compromised - Apple Support

Security and your Apple ID - Apple Support

Keep your Apple ID secure on iPhone - Apple Support (IN)

No. But if you think your account is compromised, change the password and enable two-factor authentication.