Lincoln at Essex. On the north side of the intersection

There are multiple ones between the Financial District and Government Center. All are in the middle of the road, so don't stare at them too long.

Honestly I forgot to take a specific note when I saw it. But based on my Google maps timeline it looks like I was walking from the chinatown gate to North Station. It was right next to a crosswalk in an intersection so it would be hard to miss.