The NKJV's "Hell" here translates Sheol, and the rendering materially affects how the verse is read.
Quick answer
If the realm of the dead lies open to God, the human heart certainly does. The verse argues from the greater to the lesser to make one point: nothing is hidden from Him, least of all you.
Read both lines
This is an argument, not a description of the afterlife. The first line names the two most inaccessible places an ancient imagination could reach, the grave and the abyss beneath it, and reports that both lie open "before the LORD," in plain view. The second line then closes the argument with "how much more." If the unreachable place is transparent to God, the space behind a person's face is hardly a challenge. The logic pairs naturally with 15:3: there God's eyes covered every place, and here they reach the one interior no other person can enter.
Why translations differ
"Hell" in the NKJV translates Sheol, and the NKJV's own footnote supplies that alternative. Sheol in the Old Testament is the realm of the dead, where everyone goes, rather than the developed later doctrine of final punishment; the same NKJV renders it "hell" at Psalm 139:8, where the psalmist plainly means the deepest place he could travel, not a place of torment. "Destruction" is the Hebrew Abaddon, a name Revelation later transliterates for the angel of the bottomless pit (Revelation 9:11). The proverb is reaching for maximum remoteness, naming the farthest and darkest addresses available. It is not teaching the geography of the afterlife, and it should not be made to carry a doctrine of hell in either direction.
Related: Proverbs 15:3 · Psalm 139:7–8 · Hebrews 4:13
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.