Such observations are easy for this extremely attentive animal, as it can be done from a distance while swimming out in the ocean, or inconspicuous lurking under a shady tree, due to an extremely sensitive vision, which helps it to see even in the darkest of nights.
A human intruder will not be seen as potential prey immediately, but as something unpredictable. With an arrival at a place where people are an unusual event, a human has already disturbed hundreds and thousands of animals that he or she has not even seen, the largest and most powerful of which is the crocodile. This behaviour is probably very similar to how you would act if you found someone walking around your house. We would react in exactly the same way as the crocodile by trying to find out what is going on, while cautiously standing out of view behind the curtain.