Millions leaving semi-permanent accommodation close to Barking flagged communication method (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

WIGWA m (semi-permanent accommodation) with M (millions missing or ‘leaving’) and G (last letter or ‘close’ to Barking). We had never heard of this method of communicating with flags – we only knew about semaphore.