This was not the post I was expecting from the title. Usually “battery vampires” are flashlight that can suck every last drop of energy from a battery and keep glowing the entire time. Not flashlights with bad parasitic drain.
All e-switch lights will have parasitic drain. Some are very good at minimizing it while others are not. Digging through the reviews of the 2017 K18 I see no one has measured it for parasitic drain. Draining down the battery in a day seems a little excessive though. Even worse than Nitecore’s battery killing lockout in the original TIP.
Do you have a DMM that can measure DC milliamps and microamps?