Actually it's a reflection of sunlight off the windows on the building across the alley. Just off of Lasalle Boulevard on Delaware Place.
<rant>The biking/walking path along the lakefront needs to be widened, or there needs to be another path made, so bikes can use one and walkers/runners can use the other. As both a biker and a runner, I would love to see this happen. As a biker, I would no longer have to look out for runners that suddenly cut over in front of me. And as a runner I would no longer have to look behind me everytime I wanted to pass someone. Also, Rollerbladers: YOU TAKE UP TOO MUCH ROOM, you can use the grass/sand next to the path. </rant>