If you’re chasing speed and stealth, a lock‑free concurrent hash map that hashes the keys with a secret salt is the go‑to; it’s O(1) lookups, no cache‑line flushes, and the keys never show up in the logs. For the “don’t even know you’re there” part, wrap that in a Bloom filter so you can check existence in constant time and keep the actual dataset off the surface. Just remember to rotate the salt every few hours, or the adversary will learn your pattern and the whole stealth thing will be as useful as a fire alarm triggered by your sarcasm.