It looks like it was applied from a squeeze bottle or squirt gun.
Who does something like this? and I can’t imagine they could get away with being unseen by someone in the late-night crowds, as you mention.
It's nearly always crowded there late at night... I'm not sure what it is that makes so many people visit at night.
It must be hard to have a father on that wall, and perhaps the pain is more difficult at night.