Ok. let's get some science in here about sight!
Good eyes, in good viewing conditions can distinguish separation in objects down to an angular size of 1 arc minute/0.0003 radians. But we can go further! Working out the diffraction limit of the human eye is pretty simple:
θ ≈ 1.22×λD
, where θ is the angular size of the object
, λ is the wavelength of visible light
and D is the diameter of the pupil.
Given the average human pupil size of 4mm, we can work out that the absolute minimum angular size of an object the average human eye can perceive is 0.0002 rads! Yeah, go science!!
Now to work out how far away we can both distinguish the Mavic from other objects/it's orientation/etc., and the technical absolute distance it can be seen, irrespective of orientation.
Given the formula to work out angular size at a distance: Angular Size (in rads) = Object Size / Object Distance, we can easily work these distances. The Mavic diagonal size is 335 mm (excluding props). We have to work in the same measurement for all to make this work, so let's convert the Mavic size to meters (0.335m) so we can try some difference scenarios without dealing with large numbers.
Maths Time
0.335m/500m = 0.00067 rads
0.335m/1000m = 0.00335 rads
0.335m/1500m = 0.000223 rads
0.335m/2000m = 0.000167 rads
TL;DR
The technical maximum for viewing the Mavic by the average human eye, in perfect conditions is 1675 meters / 5495 ft. It would show up as a single black pixel against the sky with no features. If we want to be able to see changes in the Mavic's orientation/view the back light we must be closer and the limit of ~1100 meters / 3600 ft would be more appropriate.
Unfortunately, our eyes degrade as we get older, and these numbers assume that you are young, with no eye problems. So yes, while the Mavic is certainly viewable past 1km, most of us would not be able to see it at 1.5km, and certainly no human can spot a Mavic at 2km.
Do with this information as you will.