While the possibility cannot be fully discounted in some cases, the chances are extremely small (smaller than any other explanation I can think up).
Why on earth would an alien spaceship travel for hundreds of years through space to take a look at our planet and not even try to say hello?
In human history all exploration has had military or economic motifs.
Either it was to seek new sources of raw materials, new trade markets or new places for the population to live. Military goals might include seeking a place to build a base from which to attack an enemy.
I do not think an alien species would have much different motifs. Trade and military action are not options over interstellar distances (unless you have faster than light drives, which I do not believe in (while it is theoretically possible to move faster than light it is impossible to get from a speed below that of light to a speed higher than that of light)) leaving only colonisation.
A ship of the size needed to house a colony (or a fleet of ships that together are large enough) would be easily detected in advance (a smaller ship spotted in the atmosphere might be a scout from one). As this has not happened (and don't tell me the knowledge is suppressed, amateurs would also see it) that too can be discounted.
I wish I were flying