I am designing an alien species that evolved to use magnetoreception to aid in navigation on its homeworld in a manner similar to some terrestrial animals. What would it take for an alien creature slightly larger than an average human to be capable of detecting the magnetic field produced by biological processes, such as a heart beating or the function of the nervous system? What is a reasonable distance at which such a magnetic field could be detected? Would it be plausible for this magnetoreception to be used for hunting prey? For the purpose of this question, encounters would take place in an atmosphere, so the aliens don't have the benefit of being submerged in a conductive fluid like sharks do.
This doesn't seem very likely.
For there to be a benefit to use magnetoreception for navigation there should be a strong magnetic field on the planet. Otherwise there wouldn't be an advantage for the initial evolution of these senses.
Meanwhile biological process magnetic fields are incredibly weak. For the brain, the most magnetically active parts of the human body, the alpha rhythm is at about 10^-12 teslas - 7 orders of magnitude weaker than the Earth's magnetic field. Brain scanners have to be heavily shielded to detect anything. Jumping the gap from magnetoreception for navigation and magnetiocreption that can detect humans might be an insurmountable leap for normal evolution.
You can conceive that maybe the planet is very iron rich, which means that local wildlife have a strong magnetic signal allowing them to be detected. And maybe humans could wear armour or something that makes them detectable.
Electroreception would make more sense Electroreception is the biological ability to perceive natural electrical stimuli. It has been observed almost exclusively in aquatic or amphibious animals, since salt-water is a much better conductor than air, the currently known exceptions being echidnas, cockroaches and bees. If you want an animal to detect humans in a similar way, I think it would be better this way, as magnetoreception is more of a biological compass rather than a prey/predator detecting tool