It seems a bug in Windows Defender causes some users to see tens of thousands of files in a folder on their C drive for no reason. Some users complain that it takes up gigabytes of storage space.

Users in the Microsoft forums start to report the issue reactively, and it seems to be a real thing:

We have an issue on a Windows Server 2019 Datacenter virtual machine with Windows Defender.
We are in: Settings -> Update & Security -> Windows Security -> Virus & threat protection -> Virus & threat protection settings -> Manage settings

When Real-time protection is turned on, after about 20-30 minutes it creates hundreds/thousands of files in this location:
C:\ProgramData\Microsoft\Windows Defender\Scans\History\Store

Most of these files are either 1kb or 2kb. Over a 24 hour period we ended up with roughly 950,000 files and it was taking 30 GB of space. This does not appear to be normal. There is no threats detected and no actively running scan or updates. These files appear to be encrypted, or at least we can't open them in notepad and see any useful data. This is only happening on one server.

Anybody got any ideas?

It happens on Windows 10 computers as well as Windows Servers, both version 2016, 2019, and 2021 R2. The bug seems to be related to Windows Defende version 1.1.18100.5.

Defender seems to deploy multiple thousands or even hundreds of thousands of files C:\ProgramData\Microsoft\Windows Defender\Scans\History\Store. One user says he sees 400,000 files, another even 950,000. These are files of one or two kilobytes in size, but together they take up gigabytes of space on disks. According to the affected users, this also causes problems with backups.

Microsoft says in the forum that it is aware of the problem. A fix will follow later on Thursday.

Windows Defender users are complaining about bug that creates tens of thousands of files