I think most secure location abroad would be Ramstein/Landstuhl/Kindsbach.
Would fit flight path info given thus far.
Landstuhl hospital is highly secure. Benghazi victims were treated there under max secrecy.
Going back to the night POTUS flew from Canada to Singapore to meet Kim, planefags were watching three aircraft that could have been AF1 over Europe. Some were on flight patterns toward AF bases in Italy. Reports coming out later said AF1 refueled in Malta.
Add to that, the incident later of the missile fired from Whidbey Island and the possibility of AF1 flying above the Aleutians on the way to Singapore..
IOW, there's going to be decoys ans we won't know shit about where it is. Europe is in darkness now, too. Visible light roughly 8-5 if weather is clear. It isn't now. I guess any landing will be/was planned for darkness.
I was looking at the Q drop that POTUS will be isolated and on AF1 and abroad when the SHTF.
Like I said earlier, most secure abroad would be Ramstein, if in Europe, IMO. I don't think anyone will know where POTUS plane is if/when this happens, so anything seen is probably a decoy.
Trips abroad by POTUS can be secret, Like Bush W Thanksgiving Day in Iraq.
Most likely scenario now is an admin honcho flying over to see what's happening in Syria, I think.