Actually...who HASN'T seen their MIL naked!? (jk
)
But yeah it happened during my first marriage. My wife and I were staying at her house for the weekend; I was out partying with friends and came back late. I snuck in quietly and saw my naked MIL walking out of the bathroom. She turned towards me and gave me a "look" before walking back into her bedroom. I was like: "oh?!"
Well it just turned out that she slept nude; and being that she knew her daughter and I were nudists, when I "caught her" naked she just figured it wouldn't be a big deal for me (especially considering she'd "caught me" naked at our home too). That's all it was (not that I figured her "look" meant anything more).