I have discovered that ArcGIS 10.3, when some geoprocessing tools (Buffer) are used creates a single digit directory "1" for some reason.
as in C:\temp\1
This new directory is empty and is produced for no apparent reason.
New install of 10.3 ArcGIS Basic license Level
put a shapefile: parcels
Created a temp MXD: temp.mxd
added shapefile to mxd, saved
selected one parcel
create 100 foot buffer around selected parcel
New "1" directory created with nothing in it.