I've been meaning to check this on my Leafs but forget when I actually have the hood opened. Well seeing this thread brought it to my attention again so I went out and checked my Leafs. Both my '12SL Japan built Leaf and '13S TN built didn't have any caps over the nut. All 4 nuts were quite rusted/corroded with the drivers side being worse shape than passenger side. My '13 currently had water sitting around the bolt, maybe 1/4" deep.
My guess is the caps were just on the '11s and I agree a good glob of thick wheel bearing grease in the whole cup should help, after things have dried out. Poor design for sure.
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a