I spied a bit
 

in the 90's before I got married, I lived in a small but 2 story apartment, dowstairs mini living area and kitchen, upstairs a bed a a bathroom basically, but the bathroom had swinging door like a old western bar, anyway, they would cover the torso of an adult; but the catch was, you could totally see inside the bath below the door, from the living area dowstairs

so, my GF and a friend spent a couple of days with me, and when they would shower I would run dowstairs to give then "privacy"

many, many years later, I confessed to the friend what I did, she laughed and called me a moron lol

A hometown friend of my wife's was going to be spending a few days with us. I enjoyed her company, she was nice looking and had a good sense of humor, she was fun to be around. She was going to be arriving while we were on our way back from a day trip, so we told her where the key was and the alarm code.

My wife was driving and she asked me to see if her friend had arrived. I checked the cams, the one in the garage showed her car was there, the one in the den showed her topless with her bra in her hand, I guess she was getting comfortable. The cams have speakers, I could not resist, "nice tits Alicia" in a HAL type robot voice. I saw her laughing and scurrying away from the cam.

So was there any follow up to that?
Did she say anything when you got back home?
Did she say anything to your wife?

it happened to me at a water park with some high school friends. One of the girls in the big pool after a slide had a tit out of her swimsuit. for a while we guys didn't say anything, but after a couple of minutes I told my friend that she had a tit out. she was embarrassed and was silent for 10 minutes. the other girls weren't with her at the time, so they didn't intervene. we were 17 if I remember correctly.

Sister in law
 

On holiday in greece with wife, her SIL and hubby.
The women were fine sunbathing in bikinis but not topless.
SiL went for a swim in pool and jumped in, when she surfaced her tits were both our. I just said "I like the puppy with the pink nose"
She looked at me wondering what the hell I was talking about, so I pointed to her tits. She looked shocked initially but then laughed as she tucked her tits into her top.
The following year we were all on holiday again, this time we hired a sailing yacht. The women had no hesitancy in going topless as just the 4 of us. SIL was first to take her top off saying I had already seen them