Sorry for your lost.
I watched the video and noticed a few things, not sure how useful they're for locating your quad but hopefully will reduce the chance of us losing the next one in a similar fashion.
Too much wind:
It looks like there was a lot of wind and the quad was having a hard time holding position in GPS mode. In fact, it is almost using all available horizontal thrust to try and stay in one place. You can tell by looking at the OSD pitch and roll angle indicator. In GPS mode, the max tilt is 25 degree (combined vector). In your case, it is at 20-21 degree just holding still, so that means that it is near the limit of the ability to maintain station in GPS mode, you'll make very little headway against the wind if you have to and FS mode would not work as it has a even lower pitch limit.
At the start of the flight, I usually lift the quad to maybe 50-60ft (above the nearest obstacles) and have it face directly into the wind on GPS mode with all stick centered to observed the pitch angle as it holds position. It gives you a clue on hold hard the quad have to work to fight the wind. If the pitch is >12 degree (50% power) then I would only ever fly it upwind just in case.
Note that the max pitch in a bit higher in ATTI mode (35 degree) but lower in FS mode is even lower at around 15 degrees, so you'll have to be very careful with that.
Descend is too slow:
With firmware V3.02 and up, the max descend rate is 2m/s in GPS/ATTI mode. So your flight is at risk even before you finished your ascend to 1.6km altitude as it would take like 13-14 min just to come down at 2m/s. Your option at that point is either go to manual mode which doesn't have descent limits, or if your TX is not so configured, do a CSC to shut down the motors, let it free fall to maybe 300 meters or so then restart and hope it stabilizes. The later option is risky, but we've seen videos of a NAZA controlled quad doing such and in your situation the alternative would certainly be worse.
Where did it fell?
As to where it would have fell after you lost video contact... I am not expert at this area but it would likely depend on what you were doing with the sticks for the next 15-30 seconds. The battery probably still have some power to not completely free fall even if it cannot maintain altitude for a little bit even after the video fails. So if they are on, it can possibly drift quite a bit. I would search downwind from the last known location.
Here is my estimates:
Estimated wind: 12m/s, based on the fact that it takes 20 degree pitch to hold position
Freefall speed (terminal velocity): 25m/s (golf ball is 32m/s and P2 has lot more to catch the air like props and stuff)
Semi-powered uncontrolled descend at: 10m/s for maybe 20 seconds more before total freefall
Last known altitude: 1200m
This would mean it will fall for about 60 seconds, putting it about 800m downwind from last known location based on our very rough estimation.
