Coming back from Canada

I just flew back from Canada. As required, I filed a Canadian flight
plan and contacted US Customs (I cleared Customs at Sault St. Marie,
Michigan). No one said anything about me having to close my flight plan
with USA flight service (and come to think of it, I never have closed
the other times I've landed). There is no flight service frequency that
I could find at Sault St Marie. Anyway, after clearing customs, I
called FSS. My flight plan had been closed! Does US Customs call US FSS
and close my flight plan? What happens if I don't show up? Will I get
Canadian search and rescue? No one at FSS seems to know. The FSS guy
seemed to think I was responsible for closing my flight plan, yet I've
never done it before either.

Anyone know how this is working?