Why am I getting a security alert about an unknown file called .rnd in the WordPress core?

In some instances, security plugins such as WordFence may generate a warning similar to the following:

Unknown file in WordPress core: wp-admin/.rnd

UpdraftPlus uses the .rnd file to generate random numbers and perform certain encryption tasks. It’s harmless and will be recreated automatically if deleted.

You can either ignore the warning or adjust your security plugin settings to recognise the file.