Unmarked means carrying no sign to identify it, or left undamaged. An unmarked car, an unmarked grave, unmarked bodywork.
An unmarked police car is the standard phrase and the whole point of it. An unmarked grave is one with no stone, usually because nobody could pay for one. Of a surface, unmarked simply means without a scratch.
1adjectiveunmarkedCarrying no sign to identify it, or left undamaged.
“Amazingly, the vase fell off the shelf and landed unmarked on the carpet.”
unmarked is a quality.
From un- + marked.
Etymology from Wiktionary, CC BY-SA