What simple marks fool the eye into seeing more detail?

I was inking a busy street scene in my sketchbook last week and ran out of patience about halfway through the background. Instead of rendering every window and brick, I just scribbled clusters of short, overlapping lines at varying angles. When I stepped back, it honestly read as more detailed than the foreground I'd carefully constructed.

That got me thinking about how much of "complexity" is really just the right visual shorthand. For me, one big discovery has been that crisscrossing my ballpoint hatching in two or three directions creates this illusion of textured depth that a single clean pass never does. Another one: leaving a few strategic white gaps in a dense area makes the viewer's brain fill in structure that I never actually drew.

I would love to hear what marks or techniques give you that disproportionate payoff. The stuff where minimal effort somehow reads as rich detail. What shortcuts has your hand figured out that your brain didn't plan?