Have thought about this a lot since and am still turned on by it.
My office on a mezzanine overlooks a storefront type window. One day when everyone else had left for lunch, a car parked in front, facing the window, about 30 feet away. The driver got out and went to the business next door but I could clearly see an attractive young woman, about 18, sitting in the passenger seat. Because I had been on this site earlier that day I was feeling a bit horny and bold and started to unzip my pants. Then I thought, 'if I stand up she could see my cock!'. My desk was in front of me but there was a gap there where, if she positioned herself just right, she'd get a good eyefull.
So, standing up, pants around my ankles, at 'full attention' as it were, I started to jack-off in earnest. I've always been a bit of an exhibitionist and this turned me on immensely. My pre-cum started to ooze and, added to my jerky breathing would have made obvious noises to anyone within earshot. Around this time I thought I detected a bit of a difference in her demeanor. I couldn't quite tell if she was looking up at me through the glass. There had to be reflections on the window. Or were there? Maybe she detected some movement and was drawn to it. Then she opened up her cell phone and started talking into it and smiled, all the while apparently gazing in my direction. When she held the back of the camera facing toward me, I kinda panicked, thinking she might be trying to take my picture or something. Was she describing the scene to a friend? Or was she just looking up a number?
So I sat back down with my cock out of view. 'What now?', I thought. If she takes my picture I'll be in one shitload of trouble. She put her phone back to her ear then. I thought, 'If she IS describing the scene to a friend, it could be almost as bad. But what if she's just pretending to talk to a friend so as to give the impression she really wasn't looking and was otherwise distracted? It would allow her to look in and seem not to be staring.' I decided to go for it. The thrill was worth it. Besides, no one else could possibly see, and if it came down to it, it would be her word against mine.
So, slowly standing again, I started the show again, giving myself nice, long strokes this time. Left hand cradling my nuts, with shaking legs I was really getting into it. I lasted about two minutes. All the while, she would glance for prolonged periods in my direction, but I really couldn't be sure if she could see me. It was sunny outside and fairly dark in my office. She brought one knee up to her chest and sort of waggled it left and right. Was she giving herself a bit of a discreet rub, not wanting to give herself away? What the hell, I'd gone past the point of no return now, only I hadn't thought about what to do with the ejaculate. I shoved the papers and files on top of my desk out of the way enough to clear a landing pad for my spunk. I don't shoot too far anymore. Looking at her, watching her smile, imagining she was as turned on by me as I was, I came. Long, groaning orgasmic pulses shuddered through me. I came for what seemed like a long time, my knees almost buckling, my eyes squeezed almost shut. Milking every drop into the pool on my desk, I fell back onto my seat, spent, breathing like I'd just run a block.
Then she did something that has puzzled me ever since; mere moments later while seemingly still talking on the phone, looking in my direction and smiling, she held up the thumb and forefinger on her free hand in the 'ok' sign, and waggled it a few times. Was that a sign that she saw the show and approved, or was she just talking to her friend and gesturing about the conversation? Did she see me after all? I'll never know. Then her companion hopped into the drivers seat and she was gone. But that was one of the most erotic moments in my self pleasuring chronicles, and typing this true story now has made my hands shake and cock wet again. Damn that was hot!