The most likely cause is diffraction from the pinhole, not a sensor defect. A pinhole is extremely small, so light spreads out and forms diffraction patterns instead of being focused cleanly like it would through a lens. In detailed areas this can show up as fine lines, outlines, or a grid-like texture when you inspect the image closely.

Your later observation of rainbow effects around bright headlights also fits this explanation: diffraction can produce colored fringes and flare-like patterns, especially with very small apertures.

So the “grid” is likely an optical artifact created by the pinhole itself. If you want to test it, try changing the pinhole size or shape, or make several pinholes and compare the patterns. Different openings should change the diffraction/interference effects.