Spray - possible. My cleaning people come over once every two weeks and clean windows and blinds as well. This vent happens to be right above a window.
The shape of the stain and the spread of the dots look a lot like a spray to me. The large stain is where the centre of the spray hit, the smaller dots are where it misted out / splashed from the centre, and the neat clear line across the middle looks like the front of the acrylic piece protected part of the back / inside because of the angle of the spray. If it’s mounted the same way up as you’re holding it in the photo, it would make sense based on someone spraying upwards towards the top of the window.
As remotelove said, some chemicals can damage plastics, and that’s what I think has happened here.