Screenshot from a secret passage in the 'Slog Goes Boom' area, a short distance before the 'Abe's Moon' cutscene.
Suddenly you wonder why Abe didn't notice it here; not quite as subtle as in Abe16's screenshot.
I'm always caught by the tiny, steadily flashing red light in the background, high up in the sky during the Return to Rupture Farms level. An early reference to Vykker's Labs, perhaps?