If you're trying to fuck with the humans though...
You're not the first to suggest that, but it just doesn't pass a smell test. I mean, why and how? And with such consistently liminal results? Always on that edge.
It's an exceedingly "extraordinary claim".
They _could_ but it's pretty ridiculous.
Why is simple. If they're monitoring us they probably wouldn't want to affect the result of their observations. That's exactly why people studying nature on Earth try not to have an impact on what they're observing. It affects the outcome and it results in bad science. Or maybe they're shy.
As for how ... we're talking about hypothetical aliens who have crossed galaxies to watch us. Speculating about how they might hide from us when they get here in an HN post is beyond futile. How the heck would anyone know?
Utterly absurd, though.
Aliens could be actively trying not to be detected. It would be quite reasonable if they're monitoring us for them to disturb us as little as possible and not to leave any evidence behind. That's how humans study nature after all.