What is the invisible unselectable entity on a supposedly empty layer? My scene has stuff on three layers.  The layer buttons show a dot on a fourth layer, one that to my knowledge contains nothing - no meshes, no cameras, no lights or anything.  If I have only that one layer active, and tap 'A' to select everything, nothing shows, just the usual grid and 3D cursor.  What the heck is existing in this layer that prevents it from being empty?
Select the suspicious layer. In the Outliner, set the Display Mode to Visible Layers:



If there's anything there &ndash; hidden or not &ndash; you'll see it there.

If nothing appears, please upload your .blend file for further diagnosis.