Is this sheet on a dashboard? If so, then go to the dashboard, then take a look at the dashboard actions. My guess is that there is a dashboard filter action that is creating this.
This is an action filter (which is a Set as well )
Since it is dynamic you will not see it under Sets
So, don't get confused' Action Filter = Filter + Set ,Just go and check your dashboard action.'
Also, once you are out of that action , it(action-set filter) will disappear from the Filter
Thanks for this information! Can you tell me how that's done? I have found some online resources on this but even with that backtracking what was done on the sheet/dashboard is tricky. Will appreciate if you can please guide me on that.
Let me try to explain , I did one thing .
I tried to describe the ACTION(Region)
So, action filer is not only a set but a Combined Set (Unlike the other sets that you create manually , those are computed sets )
I thought why ?
I again took a look at my dashboard (ACTION filter set at REGION )
So, if you consider your source as SET1 & destination as SET2
The result of your action filter will be Shared members in both the set (see above image where I can see only East region at Sheet 4
after applying Action filter at Sheet 2 )
I just tried to understand it myself as well , hope it helps , if yes , please mark the answer accordingly