You: "Stealth mode!"
Ninja: "What?"
You: "Good morning, you snuck up on me again."
Ninja: "Whateverdude"
You: "Ride on"
Saying "Ninja" might have variable reactions depending upon the other person, "stealth mode" might not.
Some riders ride dark on purpose for various reasons, so speak or don't speak for your own peace of mind, then pedal on.
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Life happens, don't be a spectator.