Charles H. Spurgeon on Psalms 139:3

Thou compassest my path and my lying down, etc. The words that I have read unto you, seem to be a metaphor, taken from soldiers surrounding the ways with an ambush, or placing scouts and spies in every corner, to discover the enemy in his march; "Thou compassest my path": thou hast (as it were) thy spies over me, wheresoever I go. By "path" is meant the outward actions and carriage of his ordinary conversation. By "lying down" is signified to us the private and close actions of his life; such as were attended only by darkness and solitude. In Ps 36:4, it is said of the wicked that "he deviseth mischief upon his bed", to denote not only his perverse diligence, but also his secrecy in it: and God is said to "hide his children in the secret of his pavilion", so that these places of rest and lying down are designed for secrecy and withdrawing. When a man retires into his chamber, he does in a manner, for a while, shut himself out of the world. And that this is the fine sense of that expression of lying down appears from the next words, "Thou art acquainted with all my ways"; where he collects in one word what he had before said in two; or, it may come in by way of entrance and deduction, from the former. As if he should say, Thou knowest what I do in my ordinary converse with men, and also how I behave myself when I am retired from them; therefore thou knowest all my actions, since a man's actions may be reduced either to his public or private deportment. By the other expression of "my ways" is here meant the total of a man's behaviour before God, whether in thoughts, words, or deeds, as is manifest by comparing this with other verses. —Robert South.