My first ever observation of Quail was visual (a short flight above a meadow) and since then I only always hear them - across various WP locations, the sound is a pretty common occurrence, but seeing them is hard. I guess they could be flushed by walking in the right direction, but I don't feel the need to do it. The same with Nightjar - I first saw it in a forest clearing after sunset, but with still some light, flying around me, sometimes very close, and hunting. Since then I always only hear them; but I heard of a guy who stumbled upon one roosting above a frequented biking path, it's just about looking well enough :) A couple days ago, I saw Red-necked Nightjar in Spain pretty nicely in flight in darkness in torchlight (found as a silhouette against light pollution from a city) and also could watch it sitting on a road near me.
Now Corn and Spotted Crake, those are birds I could hear every day if I wanted, but never saw them, despite the sounds coming from very close distance.
Birds: world 2002, WP 561 (#1 Czech WP birder!*)
* and my wife is #3