In the 40,000 Universe, there are many ways that these situations are possible.
In this particular series, the Chapter is one of the creator's own making. Not much is known about their organisation or command in particular, but there are a few hints as to their operational capacity. This means that, aside from the fact that they are Space Marines, they are not restricted in their resources.
Stealth Tech is available in many forms, such as the Tacticus Power Armour
Another variant of the Mk.X is the Mk.X Reiver or Phobos pattern. The suit’s lighter-weight ceramite and streamlined design allow for greater mobility, and its servo-motors are engineered to be completely silent. The design however sacrifices protection for stealth and speed.
Or devices that can be used to generate sound diffusion such as stummers
Stummers: The reverse of a Screamer, stummers generate sound waves to cancel out ambient sounds and noises made by moving personnel in a small area. A character carrying an active stummer gains a +30 bonus to Silent Move Tests. A Stummer typically has enough power for 20 minutes to continuous use before needing to be recharged (taking about one hour)
On the other hand, I was partially correct in that Psykers don't actually use their normal senses such as sight and hearing; instead they use their psychic powers to create an "awareness bubble", and use their ability called "witchsight".
Every psyker of every species possesses the ability known within the Imperium of Man as the "Witch-Sight" or "witchsight." Because psykers draw on the power of the Immaterium for their abilities, they can see the corona of psychic energy that surrounds every living being in realspace. For sentient beings, this means being able to glimpse the nature of any psychically unshielded person's psyche -- their psychic "aura" in ancient Terran terms. Every psyker's mind interprets or experiences this information differently based on the nature of their own cultural and mental constructs. However, the one constant experienced by all is that people whose psyches are riven by the darker emotions and motivations appear in Witch-Sight as degraded monsters, while those of the opposite persuasion appear as pure or even angelic beings.
And there are tools that counteract this, such as Psy-Jammers
A Psy-Jammer is a somewhat esoteric example of techno-arcana. These devices can at least partially disrupt psychic energy and offers some degree of protection against the dark arts of the psyker and the witch. Expensive and rare, their use is largely restricted to the agents of the Inquisition, the Adeptus Mechanicus itself and those among the Imperium's elites whose paranoia often matches their vast wealth.
All in all, in this particular series, there is evidence that this Marine Chapter possibly do work with the Inquisition, and are specialist groups that are often prepared and equipped for these specialist missions; so the representation of these particular series of events is well within the realms of possibility, in the 40k Universe.
