Used to work security for a bus company that had several hubs throughout the city - each of the outer hubs had a security booth with 1-way glass of to the side.
All in all, it was a pretty boring job. You might have 1-2 incidents a day which sucked, but for the other 7+ hours you did nothing.
Well during one of these long nothing periods, I was sitting in my booth reading a book when I notice this insanely hot chick checking herself out in my window glass (because the outside was a mirror).
I admit, at first she startled me. But once I realized she couldn't see me, well... this was just too good to pass up. So I check my door, make sure it's locked... then down go the pants. Because I had to have my shirt tucked in all the time, I figured this would just be easiest.
It was really awkward at first; I mean she was hot, no doubt - but I had a hard time getting over how devious this was; that she was standing literally less than 1ft from me and I'm sitting there pounding my salami staring at her tits...
But after a minute or so of this, I was good - had the full pumping action going on, knees bent, sweat starting to bead up on my forehead (those booths are hot under normal circumstances). I was really getting into it... when the worst happened.
The chick I was staring at, after she had fixed her shirt, and checked herself out a little bit - leaned in to check out her makeup; almost pressing her face to the glass. When you get that close, your shadow blocks the outside light enough to see in. It's still dark, but you can see in... and she did. She saw right in there and saw me.
The worst thing is, I didn't notice her noticing me - I was staring at her tits. So the whole time she's realizing I'm standing there jerking off, taking it all in - I didn't stop. I kept going.
I don't know how long she had been staring in before I shifted my gaze, seeing her seeing me - her look of complete and total disbelief; which upon seeing my realization that she could see, quickly... QUICKLY turned into outrage.
That lady started slapping the glass and screaming "HE'S IN THERE JERKING OFF STARING AT ME!!! SECURITY IS JERKING OFF!!!".
As fast as I possibly could, I pulled up my pants, tucked in my shirt - and walked out of my booth, trying to pretend that was not just what happened. Unfortunately, I have no poker face, it was plain as day - even through my huge mock-police sunglasses.
I denied it; I mean what could I do - admit it? She said she was going to file a complaint, I told her she was welcome to do so; but she was mistaken.
Luckily no one believed her. That's not to say I didn't have all sorts of people running up and pressing their face on the glass just to see if they could catch me whacking it. I of course never did that again.
TL:DR Masturbated staring at a chick through 1-way glass who leaned in and saw me jerking off. I got away with... but I didn't get away with it.