Caught in the Coffee Grind: A Slippery Slope?

I've been dating casually for a few months now. We're both in our twenties and still trying to figure out what we want from each other, but nothing exclusive yet. I work as a barista at this busy cafe downtown where she likes to meet friends or grab some breakfast on her way into work. Last week, we had an intense makeout session after closing one night. It was heated and passionate, but we agreed that for now, our relationship would remain casual with no strings attached. That didn't stop the tension from building up between us when she came in during the day. Today I saw her sitting at a table near the counter, talking to some friends. I couldn't help myself, I found an excuse to lean over and whisper something naughty into her ear while refilling her coffee cup. She giggled, blushed, but didn't push me away. That was enough for me. During a quieter lull in customers later on, I snuck out the back door with her following close behind. We ended up in an alcove between the cafe and another business down the street, she said she had some work stuff to do, but we both knew what was really going on. She leaned against a wall while I stood over her, kissing again. That's when one of my coworkers walked by outside, peering through the window just in time to see us grinding our hips together and kissing passionately. He gave me this weird look and kept walking. Now I'm mortified, what if he tells anyone? Will it come back to haunt me later? Was I wrong to act on impulse with her there so openly? Did we cross a line we shouldn't have? Am I overthinking things or is this really the asshole move here? I just can't shake feeling like this is going to come back and bite me in some way. Any thoughts?