Yes, you always wonder if they are just being polite! On my retirement, my colleagues presented me with a book full of letters from past students saying what a good guy I was. They had contacted all of the students that they could find. So, prolly 20 - 30% had moved addresses, there was a response rate of c.40-45% of the rest - and they are the ones wo are being polite, the others just stayed schtumm! Nice to hve it, of course, but it does make one think.

I'm immortal until proven otherwise