I have seen once a similar situation.... I had a scout on auto explore. at first there was a lot of open territory to explore upon, as the game progressed, more and more land being claimed, my scout litterally got stuck within only 3 tiles, between an ocean on one side, a large mountain 3 tiles away, and unfortunately, newly developed culture borders belonging to 2 diff empires, that I did not have open borders with. He was stuck in a 1x3 tiled aread, and when I tried to manually cross him thru one the territories, I was warned that would declare war. soo, I gave up, and left scout there. but it was funny, he did provide a lil bit line of sight where I wouldn't otherwise be seeing line of sight, and every now and than, Id see him move back in forth in the row of tiles, unable to proceed away.