Just opened up my Mac and found an open Dropbox folder containing thousands (?) of 1pw files

@editortom - that's quite odd indeed! If you are using 1Password and syncing your data via Dropbox, then this is your 1Password sync keychain (AgileKeychain) contents from your hard drive's Dropbox folder. To hopefully assuage any fears you might be experiencing, this isn't secret information, it's right there all the time on your Mac, in your Dropbox folder. It's also fully encrypted except for some of the metadata that's used to identify the item when search functions are called. But the actual data itself is an encrypted string, protected by your Master Password.

That doesn't answer the question of why this folder was open on your desktop yesterday when you opened your MacBook, but that's not something we can help you with. There is no function in 1Password itself which would open that folder without user intervention. However, anyone accessing these items on your own local machine will get no more than they would if they had access to your Dropbox online -- just the encrypted items, plus some metadata on each item.