If you are sure the Outlook search results are incomplete, please try to rebuild your windows search index or reconfigure Outlook profile or reinstall Outlook.
In this simple guide, we are discussing that the Outlook Search results are not really incomplete.
If you search a calendar then sort by date, you are aware that some items are missing from calendar search results.
Try scroll down, till the bottom of the search results, you might find there is a group named Recurrence: (none): xx item(s) which can include some missing items from the top of the search results, now right click on the column title
Click on View Setings
Click on Group By… button
Select (none), Click on OK, OK
Restart the Outlook.
Do the same search on calendar, all results will be listed in one group, you should be able to find the missing ones.
If still not fixed, items are still missing without groups, refer back to top of the guide.