Hi Tony,
I researched a lot and found some useful methods to find a particular subfolder in Outlook, for your reference.
Method 1:
All the folders and subfolders will be displayed in a list in "Folder Size" and you could find the path of your Subfolder in this list like below.(File>Cleanup Tools>Mailbox Cleanup>View Mailbox Size>Local Data>Subfolder)

Method 2:
If you still know an item in that folder, Advanced Find tool may help us find this subfolder. Please check the steps below:
- Search the item in the search box within All Outlook Items
- Double-click to open the message you find
- Press CTRL+SHIFT+F to open the Advanced Find
- Click Browse and you will find the subfolder

If these methods above still could not meet your requirements, please check
this article for more information.(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
Hope this can be helpful.
Regards,
Jeff Yang
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