Searcher:
Very interesting find. All the times I have read that account I somehow always assumed they were in the same place until the end of chapter 17. I wonder if John 17:1 ("[Jesus] lifted up his eyes to heaven . . .") might be an indication of an outside location?
Looking at various commentaries, they seem to accept the possibility (of the group leaving the upper room after chapter 14) without being dogmatic about it. Some suggest possibly a location passing through vineyards or near the temple entrance (where there was a golden vine hanging over the entrance) whilst giving the vine illustration of chapter 15.
This is the only recent WT commentary I could find on it (Greatest Man book):
*** gt chap. 116 Preparing the Apostles for His Departure ***Also see w90 8/15 p.8 which the Greatest Man book account is taken from.
Following the memorial meal, Jesus has been encouraging his apostles with an informal heart-to-heart talk. It may be past midnight. So Jesus urges: “Get up, let us go from here.” However, before they leave, Jesus, moved by his love for them, continues speaking, providing a motivating illustration.
Bobcat