The child we've caught them off is family. OH's 4yo sister. MIL came to stay for a week, and said nothing the whole time. There are no other candidates from which we could have caught them, MIL sports a shaved head and DS hasn't had contact with his usual playmate in a couple of weeks, just her.
A heads (no pun intended) up would have been bloody nice. By the end of the week DS was scratching and I checked his hair, couldn't find anything and put it down to him having got a bit of sand in his hair the day before, now understanding the strange embarrassed look MIL had.
I'm now facing the two-week nit purge on mixed-race hair and my own. Lovely jubbly. 