What is the request "GET /?-s" supposed to mean?

I looked around the forums here, this seems to be the most suitable for asking this question.
Please let me know, if there is a better way.

I was checking the logs of my webserver and found a few GET requests for "/?-s".

"/" is the root of the webservers file system and will serve the index.
At this point I have no use for search parameters (initiated by "?").
Therefore my server will ignore the rest and simply serve the index.

The search parameters supposed to be name, value pairs separated by "&".
A made up example of a resource using such parameters could be something like:
"/search.php?search=bear&sort=descending",
depending on what that fictional search.php would actually support.

In my case there should not be any links with "?-s" around anywhere,
nor do I know how or why someone got to try and use that.

Can anyone explain about that?