High in the sky

I took my drone out this evening.
Together with Richard, I went to a place I've been to lots of times, the Emmapolder, Wadden Sea.
To our surprise, we saw a ship in the mud. I provided us with an instant point of interest.
While I was flying 700 meters away, and about 60 meters high, I suddenly got a warning on my controller "obstacle avoided".... what?? 
At that same moment Richard, standing a few meters away from me got the same message. I turned out our drones almost collided in mid air while we were looking for compositions.
Despite the fact we constantly inform each other about the heights we're flying.
Luckily, nothing serious happened. The sensors worked just fine.