Unknown user deleted my file from a shared folder

The folder that this MP4 video file C0002 was in had one other person than me in its member (edit) list.  But none of the persons are known to me and I have added neither of them. Another mystery is that these two persons do exist in my town and both are connected to a nearby University.
So how could this happen? How and when did these two persons get access to my folder? And how could one - not even in the folder member list - be able to delete this file?
Can there be a Dropbox security issue?  Or that the person deleted a file with the same name in his own folder but Dropbox accidentally deleted the wrong file?

Hi there Staffan V - sorry to hear about this. Did you share a link to the folder in question with those people or even a computer where you were signed into your Dropbox account at some point perhaps?

You can have a look at your security page and see if you can find any devices or web sessions you don't recognize.

When it comes to the folder actions, you can take a look at the events page for more information. Are you able to find anything there since that would be the place where all uploads, edits, and deletions are normally logged?

Staffan V wrote:

Can there be a Dropbox security issue?  Or that the person deleted a file with the same name in his own folder but Dropbox accidentally deleted the wrong file?

Its simply not possible that the files are linked to another account - just think if that's how Dropbox worked how many people would have issues with files like CV.doc etc. 

If somebody shows on that Share list then they have been added to it. That could be by you, or, anybody else in the share. Its also worth remembering that names are almost immaterial here because they could have changed emails etc. in the time since you added them. For example you added Person A with a GMail account. If they then join a Uni or work place and update the email/name on the account to get the Uni paid for space that would then replicate through to the share. 

Check www.dropbox.com/events to see who added who and when. 

Hi Walter. I haven't shared this folder to anyone. Look at my attachment (if it´s there), Events print screen. This Berth-Arne [removed per Community Guidelines] deleted my video file and then removed himself from access.  It seems to be a sabotage. 

Hi Mark. Se my reply to Walter.

I have never given any access to this folder to anybody.  

I have never added these persons Berth-Arne [removed per Community Guidelines] and Sofia [removed per Community Guidelines] to this folder or my Dropbox at all! I don't know them.  

I have never given any access to this folder to anybody.  

That folder doesnt show when Berth-Anne joined. Note that you HAVE to have given access to the folder at some point for it to be a share. It is impossible for it to be a shared folder without it being, well, shared. 

The only way people can get access to a share is via being invited. So, if somebody was in the share and then left and you suspect sabotage they have accessed it via your account - i.e. they have got on to your account, joined, deleted and left. But, lets be honest if they had access to your account why would they join as a share and then delete? They wouldn't, they'd just delete. 

The other thing to just point out here is they could have just moved the file out of the shared folder to another location such as their desktop which would then delete from Dropbox. That is very likely, and common, for people who dont know Dropbox.