It is not currently clear if I am looking at a map that is showing all the available data i.e. there could be a definition query applied to one or more layers. An indicator of a layer having a definition query applied would be useful in the Contents Pane.
I encounter this need when checking maps that may have had a definition query applied for a previous purpose but I am unaware of this. I manually have to check layers for definition queries being applied, but an indicator would be really helpful to see immediately which layers are being filtered.