The first stage was noticing the bit of yellow machinery not far from the first two tutorial sections (for dots and B/W squares). At that stage I did actually try to activate it, but due to not quite fully understanding the mechanic, it didn't work.
Following this stage I did notice it a few times as a recurring motif, (most notably the window in the village - next to the dock - looking on to the orange shipping crate) but dismissed it as, well, a recurring motif.
The second stage was the river view from the mountain. I
did register that the panel looked the same as the river, but I came to the conclusion that activating the panel had made the river flow... for some reason (If you look back through my comments in the thread, you'll see that in there