re: Why would they remain hidden?
I have to disagree. There may be good reasons for them to remain hidden. If they were to visit earth for 'scientific" reasons, (anthropology, biology, who knows what?) they would want to disturb the system under study to the least possible extent, i.e. remain hidden. If they were on a mission of conquest or foraging, the less we know about them before they strike the less troublesome we will be. Either way they would be cryptic.
If however earth is just one more rather dull inhabited rock in space, they probably just want to put a tick against our planet as part of a cosmic biodiversity project. Zoom by, take a few snapshots and off somewhere else.