Hi David,
I understand that the Outlook v15.6 search doesn't work well. Because this kind of issue is related to the Outlook client while our forum focuses on Office 365 Online services with limited resources on the Outlook for Mac client questions, I recommend you post this question in the Outlook for Mac forum below for professional suggestions:
http://answers.microsoft.com/en-us/mac/forum/macoutlook?sort=lastreplydate&dir=desc&tab=Threads&status=&mod=&modAge=&advFil=&postedAfter=&postedBefore=&threadType=All&tm=1405580497543
Your understanding is appreciated.
Regards,
Xinyu