This would seem to be an oversight on Google's part. Reminders don't appear to be searchable.
When you use the advanced search options, you can limit your search to a particular calendar (including "Other" calendars you've connected) or "All Calendars", but there's no option for "Reminders".
So it appears that there's currently no way to search for Reminders in Google Calendar.
As suggested at the Product Forum, you should use the "Feedback" tool to let the development team that this crucial bit of functionality is missing.
For what it's worth, if I search my calendar on my Android device, it does find Reminders. (It also works in Inbox by Gmail on my Android device.)