Imagine a human observation and research facility being built on a planet in a distant solar system.
How could an alien colony exist beneath the planet's surface, staying completely invisible to humans on the ground? Let's say that aliens have a portal technology allowing them to travel from within the planet's insides to their home land at will and transporting resources back and forth - without ever touching the planet's surface.
Let's say their main goal for having the underground colony is resource extraction from the planet, and they want to stay invisible to avoid any conflicts if possible.
Could an alien colony be shielded to make it undetectable to human equipment either from afar and from up close?
Let's assume that the alien underground colony was built before humans came to the planet, so that main alien construction works were already done at that point.
Could it depend on passive sound and EMP proofing? Could it incorporate active silencing devices? The aliens still need to perform mining and ore processing inside of the planet, so that normally would be detectable by human equipment.
How could that be prevented? Using energy based mining tools instead of impact-based ones?
This question was marked as a potential duplicate of another one: How big could an alien-made object on the far side of the moon become so that discovery still can only happen by accident?
Though I think it's different, as the linked question asks about objects on the surface of the moon, while this question is about covering alien mining activity under a planet's surface.
To give more insight in my plot idea:
Humans have built an observation and research facility in a distant solar system - not knowing the planet is being mined by aliens, who have built a stealth outpost deep under the planet's surface. After a few years of operation the humans started drilling into the planet for research.
An alien colony hidden beneath have realized they cannot hide there for much longer without being discovered so they decided to attack, until they still have the upperhand of being unexpected - as evacuating and hiding all their traces was impossible to do.