When I reached level 41 (don't know if that had anything to do with it or not), I was thinking of giving Lydia some different armor, and when I went into her inventory list all her steel armor was gone: gauntlets, boots, chest armor, everything! She had all the ebony stuff I had given her, but not the steel. Anyone else have this happen?
Default NPC gear is not viewable without mods.
You can transfer gear and equipment to followers and they will automatically upgrade if it is better for them to use.
A related topic is here companions and armour