The error indicates that something is trying to retrieve the given resource, but the resource does not exist in the given location.
The could be several causes for this.
It could be that the css is included in for example index.html or login.html, resouces that can be found in your theme folder. It could also be that one of your widgets is trying to load it, It can be tricky to pinpoint that.
But i also see a lot of seemingly random retrieves nowadays which are caused by vulnerability scanners: scripts that roam the internet to check if sites are vulnerable to known security vulnerabilities. That could also cause these errors in the logs.
In your case, i would suggest doing a file search for the text “open-sans.css” to see if you can pinpoint the resource that is including the given file.
As a last resort, you may even consider placing an empty file with the given name in the given location, just to get rid of the log message. But that is a bit dirty of course.
I hope this helps.