'subtle mark' is correct and usable in written English.
You could use it to refer to a slight indication or impression left behind, such as a subtle mark in the snow where a deer had recently been standing. For example, "The only sign that the deer had been there was a subtle mark in the snow."
Humans and their livestock are increasingly medicated and disinfected, and all those drugs and chemicals are leaving a subtle mark on the country's waterways, government scientists found.
Six riders in a line, moving uphill and into the heat, south in a still field, knobby tires on dusty earth, leaving a subtle mark in the wild silence.
The current design does not differ from other California licenses except for a subtle mark on the front and a disclaimer on the back in small print: "This card is not acceptable for official federal purposes".
Even short of that, sightseers managed to leave a variety of subtle marks.
The only real students we admired were the ones who, like us, failed to hide the not so subtle marks of poverty.
The distinction, although subtle, marks the difference between high- and low-level learning, and it speaks to a central question of cognition research: How do you determine whether an animal is thinking through its actions, or simply learning through association a series of behaviors and combining them?
Against a baseline of common experience, a factor that might be distinguishing (the short stature, the age and frailty, the profusion of Fighting Irish paraphernalia) drops away to reveal subtler marks of distinction.
But those who do take notice of the arched passageway, marked only by a subtle sign, will find a courtyard café whose backstory belies its surroundings.
But there is a more subtle question mark about the credibility of being a perpetual straddler.
In keeping with Maxwell's signature minimalist style, the collection is devoid of distracting patterns except for a single subtle signature mark on the rim.
Whereas a sculpture by Michelangelo reveals subtle chisel marks, the robot's marks are heavily grooved, patterned and almost futuristic looking.
