If it tends to rain where you live, you want to fix quickly- at least
secure a tarp up there. Since you don't know when they came off
(probably in high wind) try to check inside to see if there is water
damage- dark stains on rafters bebeath missing shingles,
bubbles/stains in ceiling under any possible leaks.
If it is really just a few shingles missing, it shouldn't be too
expensive to fix- and estimate should be free. No idea if your
warranty will cover this. I'm wondering, though, about the condition
of your roof- you may find that it is deteriorating and in need of
replacement.