How Can a Standard User Remove Their User Name From Users?

My flat-mate had a standard account on my PC (I am an Administrator) and somehow this morning he deleted his name from User Accounts. I then had to delete all the folders created under that user name.
How could he do that & is Windows not so secure as one thinks?

I assume you mean that his username does not appear in Control panel, User accounts

Is his username still listed in the response to this command?

Code:

Net user

All the best,
Denis

No it doesn't as shown here:

I think you have to assume that your flatmate either found out your Admin password or used hacking tools to achieve Admin permissions.


Denis

Try3 said:

I think you have to assume that your flatmate either found out your Admin password or used hacking tools to achieve Admin permissions.


Denis

Yes, I think you are right. I've changed my PIN just in case.

Ex_Brit said:

I've changed my PIN just in case.

And your password, I hope.
Do consider putting a lock on the door to the room that your computer is in.
Have you asked your flatmate how he removed his user account?


All the best,
Denis

Try3 said:

And your password, I hope.
Do consider putting a lock on the door to the room that your computer is in.
Have you asked your flatmate how he removed his user account?


All the best,
Denis

It's in a common area so will secure it the best I can. Malwarebytes Pro and Windows Defender don't detect anything unusual.

Try3 said:

And your password, I hope.
Do consider putting a lock on the door to the room that your computer is in.
Have you asked your flatmate how he removed his user account?


All the best,
Denis

Also set a bios password as a very common way to hack password is to boot from usb drives with hacking software.