You can be safe without being able to see your craft IMO, as long as you can see the air space your craft is located. You'll always need LOS to have a good connection to control the craft, so you should always be able to see the air space the craft is in. Beyond 2500' I can't see my craft if I take my eye off of it, however I can hear and see other manned craft if they approach my airspace. If that happens, and it has, I simple go from my normal 150' AGL down to about 50' AGL. From there I locate the craft with the drone camera until it leaves my airspace. The noise of an aircraft (helicopter or plane) is audible for miles. At our altitudes the most likely encounter would be helicopters, and those have a distinct sound and generally move slower than small manned aircraft. There is no concern for jet aircraft if you're beyond the 5mi radius guideline for airports. You can see and hear a helicopter for miles and easily determine how to avoid conflict way before they arrive your flight area, assuming you're paying attention to your air space. That said, if one of those silent UFO's show up in my airspace suddenly, emerging from a worm hole, I'm screwed, but I yearn for that mishap to occur someday
. Epic video potential. At least I know aliens won't be suing me in court for taking down their craft.
The more difficult situations are silent manned aircraft, like para-gliders and ultralights that aren't as loud, but are highly visible. This is where the assessment of all new flight areas is important, looking around the area to size up the surroundings, along with any potential air traffic. A couple weeks ago I was flying in an area that had para-gliders launching from a cliff on the Hawaiian coastline. They launched at about 300' MSL and flew at around 400 to 500' MSL. I watched them for about 30min to determine their flight patterns and then launched my
P4P. I was no risk to them. Since they move quite the
P4P is way more maneuverable than they were, I could easily evade conflict needed to avoid conflict, but I never had to evade them as it turned out. Simply by staying at lower altitudes (about 100' above the ocean) where they didn't fly, and keeping a good quarter mile distance away horizontally, there was no risk of conflict. I also had assistance of a spotter, keeping track of the para-gliders. Some pilots may not be comfortable with that, and that's good. If you aren't confident to fly safe you should stay grounded.