In 2017, I survived an acute illness which required me to convalesce for six months under the casual supervision of family. Graciously, my widowed Mother-in-Law (MiL) volunteered. My wife decided to stay as well, in support, so we left our home in the care of a trusted neighbour and moved in with my wife's mother temporarily.
Whilst my wife was away at her job during the day, my MiL and I had the house to ourselves. Most days were very painful for me and I couldn't accomplish much. Usually, the painful days prevented me from performing basic hygiene. But occasionally, I had some decent days. When the occasional good day occurred I showered, et al, immediately.
Unbeknownst to me at the time, the bathroom door failed to remain latched when subjected to steam from a long, hot shower. It had the habit of swinging open at the most inopportune times. I was showering unaware that my MiL could see me through the semi-transparent shower curtain as she passed by the hall. It never occurred to me, until much later, how frequently my MiL would pass by every time I showered.
One of those early "decent" days stands out in particular. After showering I hung up the towel to dry and stood naked at the sink, somewhat diagonal from the door, performing the usual grooming regimen. I felt a draft, thinking it was the AC kicking on. A shadow passed by and I heard an unmistakable (almost shrieking) gasp! My MiL stood frozen in the hall, peering around the doorway, gawking at my tackle. I then realised the door was ajar and apologised. She scurried off and wouldn't look me in the eye for the entire day.
The adrenaline rush from this was unreal and completely surprising as I'd never entertained the notion that I might have an inclination towards exhibitionism. Of course my reaction was easily exacerbated as my MiL looks like she could be my wife's twin sister, except twenty years older. Additionally, she definitely exudes an
"aloof intellectual cougar" vibe.
For several months after, there were many feverish days where all I could tolerate wearing were a pair of boxer shorts, t-shirt, and slippers. I wasn't intending to be lewd, it was simply an attempt to remain as comfortable as possible during uncomfortable periods of recovery. It was during these days, I began to notice a change in my MiL's behaviour. For example, we often conversed on the sofa together and I caught my MiL staring down at my boxer shorts with increasing frequency instead of looking me in the eyes. Whilst passing by in a corridor, she would habitually glance at my crotch each time, and a few times I noticed her seemingly ogling my backside as I limped away.
If there were any self-doubts about my knewly-realised exhibitionist streak, such doubts quickly vanished with a rather thrilling occurrence near the end of my stay at my MiL's home. As she left for work, my wife happened to leave our bedroom door ajar. Not long after, I got out of bed to change out of my pyjamas. This was starting out as one of my more painful days, so I simply let things drop to the floor. Standing naked, I was attempting to put on boxer shorts without bending over much resulting in a bit of a futile foot shuffle, when my MiL appeared at my bedroom door to inquire about something unremarkable. I stood still, she stared. She then made an abrupt about-face, legging it down the corridor.
My heart was racing like a rabbit's at this point, as I was reveling in the thrill of it all. Then my MiL re-appeared near the door to continue speaking with me, obscured behind the door-jamb seemingly to give me privacy. I quickly noticed she could clearly see everything in the reflection of a mirror on the bedroom wall next to me. She was positioned perfectly, so I turned towards the mirror to give her a good look in the reflection and continued conversing with her as nonchalantly as possible, pretending to forget about my sartorial circumstances. I was about to ask her for help getting dressed
(wink, wink 
), as I was slowly growing semi-erect. She abruptly declared she would let me finish dressing then departed again down the corridor.
There were a few more encounters with my MiL during our stay with her, but nothing noteworthy.