Collins Dictionary labels variable nouns. (Note the last example sentence)

They are prepared to hand back a hundred prisoners of war a day as a sign of good will.

They are prepared to hand back a hundred prisoners of war a day as a sign of good will. friendly.

His face and movements rarely betrayed a sign of nerves.

His face and movements rarely betrayed a sign of nerves.

His face and movements rarely betrayed a sign of nerves.

Your blood would have been checked for any sign of kidney failure.

Your blood would have been checked for any sign of kidney failure.

Your blood would have been checked for any sign of kidney failure.