I dunno about a debt collector, I wouldn't be worried about somebody knocking in the door certainly..

However .. I'm recently in CH from the UK and trying to sort out child benefit so that we can start getting paid it here. I know that both sides share information, like if you got paid X in the UK then you'll get Y in CH to make up the total that you're entitled to.. Same sort of sharing goes on between both child benefit office and HMRC .. I spoke to HMRC a few weeks after arriving here and they mentioned what sort of benefits i was getting from my employer in CH - like medical allowance etc. So much for secrecy.

Bottom line. they might get the money back somehow, like from whatever you might be getting here.