Dark Eldar are sneaky sadistic space pirates. If they don't want to be seen or heard, with technology or with human senses, they won't be detected. They can show themselves with one blip to terrify their prey before striking: it's sadistic, evil, sneaky and right up their alley.
One thing you should keep in mind is how this is going to end up for the players: will the Dark Eldar sneak up on them and strike without them having a chance to react? If so you might want to reconsider. But if it's merely foreshadowing it could certainly work. Or consider working this into a Space Hulk scenario: there are more blips than bogeys, meaning the players have to be on their toes at all times.
If you want to know things like this, just put yourself in the mindset of a psychopath space elf in a gimp suit and ask yourself if you would do that to terrify your prey before striking (but please do not use this mindset to hurt acual people).