If they were so advanced, they probably wouldn't need to get close enough to Earth for us to spot them, in order to study us. And if they did go to the trouble to come all the way out here, even entering the atmosphere, don't you think they'd stop in and say hi...or kill us all?
People who are Nature Observers TYPICALLY do not disturb the animals they are watching in the wild. In fact, they'll often camouflage themselves in ways that the animals would not detect them. The reasons are two fold. 1. People want to see the animals acting "natural" without outside stimulus, and 2. People don't want to risk being killed by creatures capable of killing them.
Now, there are exceptions to this, but by and large, a pure biologist isn't going to walk up to a giraffe and say "hi".
If we are on the same scale to an alien, as say, a lion is to a human, who's to say what they will or will not choose as a method of interaction?
And any species that's crossed the fuggin Universe to visit or research us, (i.e. has the ability to travel at warp speed, and correctly calculate the solar system's movement within the galaxy and the planet's movement within the solar system) ain't crashin' his damned space ship. I'd wager a large sum of money on that.
While it sounds unlikely, we can't make assumptions of the possibilty of this happening because of the wide array of unknown factors. Perhaps their technology is reliable for travel in a vacuum, but in an atmosphere there are problems? Perhaps they had a malfunction? Our airline industry is the BEST in the world, and we still have accidents.