That kind of discreet exhibitionism is right up my alley. I'm generally too circumspect about it to actually allow a stranger to see my goods, no matter how innocently it could be posed, but I've often let that happen with people who I know would not be offended. About the only time I "flashed" a stranger in that manner was during a road trip when I stopped at a Sonic to get a burger. I noticed that the cute young lass who was delivering the food to the cars would stick her head in the window of the car a little ways to make the exchange. So I adjusted my leg and my shorts to where there would be a clear view of my dick and balls in the opening of my shorts leg, but such that it could be believable that I was unaware of that fact. Sure enough, when she skated up to my truck and stuck her head in the window she paused for an indelicate length of time staring down my shorts. Then she smiled, handed me the bag and happily accepted my payment with a generous tip. As she skated away she turned slightly and waved goodbye, so I guess she wasn't offended. I felt kinda guilty after that, but I have to admit it was a thrill.
But my most memorable such experience with strangers was several years ago at a public park with a bunch of people strolling about, throwing freezbies and walking their dogs. I was sitting on a park bench by myself and enjoyed the feeling of letting my genitals slowly slip out the pant leg of my shorts. Whenever someone would approach closely enough to catch me in the act I'd either turn a little on the bench so my leg would be blocking the view of my dick and balls or adjust my shorts so I was covered. It was a unique thrill for me to alternately look down at my exposed genitals and then look up at all the people around me. Finally, when no-one was anywhere near me I pulled my camera out of my pocket and took some photos that I'll attach here. To this day I get excited just looking at those pics and remembering that day in the park, which more often than not leads me to a masturbation session.