Did you just stumble upon a download or a file on your computer that has been digitally signed by Xamarin Inc.? If so, please read on.
You will typically see Xamarin Inc. when running the file. The publisher name is displayed as the "Verified publisher" in the UAC dialog as the screenshot shows:

You can view the additional details from the Xamarin Inc. digital signature with the following procedure:
Here's a screenshot of a file that has been digitally signed by Xamarin Inc.:

As you can see in the screenshot above, the Windows OS reports that "This digital signature is OK". This implies that the file has been published by Xamarin Inc. and that no one has tampered with the file.
If you click the View Certificate button shown in the screenshot above, you can view all the details of the certificate, such as when it was issued, who issued the certificate, how long it is valid, etc. You can also see the address for Xamarin Inc., such as the street name, city and country.
DigiCert SHA2 Assured ID Code Signing CA has issued the Xamarin Inc. certificates. You can also view the details of the issuer by clicking the View Certificate button shown in the screenshot above.
These are the Xamarin Inc. files I've collected, thanks to the FreeFixer users.
| Detection Ratio | File Name |
|---|---|
| 0/61 | mDNSResponder.exe |
| 0/62 | mDNSResponder.exe |
| 0/58 | XamarinCleanup.exe |
The detection percentage is based on that I've gathered 181 scan results for the Xamarin Inc. files. 0 of these scan results came up with some sort of detection. You can view the full details of the scan results by examining the files listed above.
The analysis has been done on certificates with the following serial numbers: