lie in wait, spy on, reconnoiter
窺 centers on observing secretly or from a hidden position: lying in wait to watch, spying on someone, or reconnoitering an area without being seen.
窺 combines 穴 (hole, cave) with 規, which likely contributes the sound and the idea of observing or measuring. The compound suggests peering through a hole or from a hidden place to watch.
The top 穴 is a hole or cave. The bottom 規 suggests a rule or standard. Imagine someone hiding in a cave, peering through a small hole to spy on others and measure their movements according to a secret plan.
For キ, picture a spy lying in wait with a key to a hidden room: key -> キ, and they use the key to unlock a vantage point for spying.