"I can tell that a test-tube Krogan used to hang around this general area a year ago by touching the floor."
It's magic.
I disagree, but its probably just my knowledge of biology that allowed me to think of a way it could be possible. I wrote a long post months ago about the probable nature of thus Prothean sensory ability. It is extremely similar to olfaction. Contrary to popular belief, it is actually olfaction that conveys the most detailed information about the environment, which is why most species utilize it extensively in one way or another. For example, we now know that dogs can detect the equivalent of a teaspoon of sugar in an Olympic sized swimming pool. They have the capability if not only smelling cancer on someone, but specific kinds of cancer. They detect olfactory information regarding individual identity, sex, social status, mating status, fear, all from chemical signals left in urine and feces. They can even smell oncoming diabetic crises and seizures. Just to use one single, exception animal as an example.
Thus, it is entirely biologically possible that a species with human level intelligence could detect extraordinarily complex and intricate information via olfaction. Can you imagine how different a visit to the doctor would be if they could simple smell that you had cancer right off the bat?
And so, perhaps this is what Javik is doing, but rather than having olfactory receptors in nasal epithelium, he has them on skin. This is potentially supported by the fact that he compulsively washes his hands after touching anything, thus removing all molecules from olfactory receptors.
It is plausible, not magical.
Edit: Ugh phone autocorrect. Too lazy to correct grammatical errors, sorry.