There are a group of individuals asking to join fleets, once they are invited they check out your fleet bank and If they find your bank isn't protected they clean it out of whatever items they can get.

My fleet bank is locked down so the person who joined us found they couldn't steal items then promptly left us.

Here is the M.O. as I have noticed it....
1. They are typically at a base bank terminal when asking for invites
2. If you have any open permissioned tabs they take what they can
3. They suddenly quit
4. They block any private messaging

I'm guessing older fleets (like mine) are well aware of this but smaller and less established fleets may not be as informed.

Please make sure, that at the very least, you lock down any items of value to your fleet!