If you wanted to disguise a probe, it seems the ideal form would be a black body, not to disguise it as an asteroid, I would guess. Unless they're playing mind games.

Anyways, given all the EM radiation we're tossing out into the void, some species' probe out there is going to pick up our existence sooner or later.

And at ~395 earth-moon distances it's difficult to imagine how effective a 400-meter object, disguised as an asteroid, would be at seeing much detail on earth.

But it is tantalizing and fun to imagine--the appearance of a gravity boost... Flat surfaces... Near to earth... Lack of any ice or outgasing, a solid object, many characteristics not seen in other known-asteroids, rotating on its long axis... :)