Hi Aquilles, This is Ramesh.
4.18.24080.9-0 could be Microsoft Defender's latest platform update folder. It's located at:
"C:\ProgramData\Microsoft\Windows Defender\Platform\4.18.24080.9-0"
I'm not sure what caused the notification during the shutdown.
If you're concerned, you can use Malwarebytes (Free) as a second-opinion scanner. Download Malwarebytes Antimalware, run a scan, and eliminate every malware it finds.
https://www.malwarebytes.com/mwb-download/thankyou
Note: the Malwarebytes setup may activate the Premium trial (15-day) version. After 15 days, it switches to the free version automatically.