Never paid my attention to the notion of "archived" messages in Gmail - which I presume means just "stored but not immediately visible". Not sure I see much value in that designation - but be that as it may, just last night I archived one by mistake and then didn't know where to look for it.
Turns out it can all be found under the "All mail" umbrella in the labels/categories pulldown on the left. Details over at StackExchange. Hat tip to all who had done the research.