Charles Ellicott Commentary Psalms 139:3

"Thou searchest out my path and my lying down, And art acquainted with all my ways." — Psalms 139:3 (ASV)

Compassest. — There is some obscurity about this word. The Hebrew verb primarily means to scatter, and is used for throwing grain about to winnow it (Isaiah 30:24; Jeremiah 4:11; Ruth 3:2). Therefore, by an easy metaphor, it may mean to sift or search out. The Septuagint and Theodotion, followed by the Vulgate, have traced, investigated. Jerome has winnowed. The Authorised Version rendering appears to come from a mistaken etymology.

A most plausible suggestion connects the verb with zûr, to lodge, which makes a perfect parallelism with the verb to dwell, in the next clause. Literally:

About my path and bed you are a guest,
In all my ways you dwell;

i.e., you are as familiar with all my life as one inhabiting the same house could be.

My path. — Literally, my going.