We've had several customers report to us that Vaultpress (a WordPress hosting company) thinks that Normalizer.php contains suspicious code. There could be other hosting companies that report this too, but this is the only one we know about.
If this has happened to you, you have nothing to fear and you can just ignore the emails.
Normalizer is a core PHP library found in many WordPress plugins. Here's some more nerdy information about it if you're interested: https://wordpress.org/support/topic/vaultpress-detected-two-codes-as-a-suspicious/