Hello anonymous user
The device show is a Hybrid Azure AD Joined device, meaning it was joined to on-premises AD and Azure AD. The activity timestamp is triggered by an authentication attempt of a device when:
- A Conditional Access policies requiring managed devices or approved client apps has been triggered.
- Windows 10 or newer devices that are either Azure AD joined or hybrid Azure AD joined are active on the network.
- Intune managed devices have checked in to the service.
Just because it is deleted from Active Directory, doesn't mean you have removed the device. Follow the instructions from this page here to correctly clean up devices. https://learn.microsoft.com/en-us/azure/active-directory/devices/manage-stale-devices
I hope this helps provide you with the information you need. If it does, please make sure to mark the question as answered so it helps other people in future.
Kind regards
Alistair