(second attempt to post this question)
We have a mix of Win7 and Win8 computers, with ArcGis 10.2.1
On the Win7 computers, under Add-In Manager > Options tab, we can add a well-known folder to the box "Search for additional Add-Ins in these folders". This path is retained by ArcMap, (possibly in the Normal.mxt since it is present in a blank ArcMap doc, before opening an mxd).
This path is not retained on Win8 machines. When you close ArcMap and reopen it you have to reenter the path and reload the add-ins.
Anyone encountered and solved this?