Someone opened some old cryptocurrency files on my Dropbox, how?

Hello,

As you can see from this screenshot, someone opened some files in my Dropbox, all these being old cryptocurrency wallets (there was nothing inside fortunately).

If I check from the device log (settings-security-devices) I am the only one logged into Dropbox, from my desktop and from my Pixel phone. The last activity from my Pixel phone is set as 22h ago, same as the time these wallets have been opened. 

Now: I know for sure that I didn't open any of these files since I even forgot that I have them. And I don't have any cryptocurrency app on the phone that is connected with these wallets. 

How do I find out who and how they opened these files?

PS: I have 2fa activated on Dropbox since many years. No one has physical access to wither my desktop nor my phone.