You're correct, Andrew. Page 508 of The Story of J. M. B:
"Barrie, however, had the best part of a week only just behind the lines, in the middle of July (1917). He climbed the Messines Ridge, or what remained after the terrific explosion which had led to its capture last month. He saw balloons being brought down by German aeroplanes, and their observers floating to earth. He managed to get forty-eight hours' leave for Peter. And - this time in very considerable danger - he sought and finally discovered George's grave, among thousands of others, near Ypres."