Bilbo may not have told anyone about going away, but Gandalf did lock the door after him. (At least that's heavily implied.)
About one year is not a very long time. What kind of bizarre society is Hobbiton where you have to constantly be on your guard and show yourself every day outside or else people start breaking into your home and presume that it's abandoned and go through your stuff and sell it off for nothing?
It frustrated me that Bilbo went so far as to pay for his own stuff to get it back. I cannot express in words how furious I would be if the same thing had happened to me, especially after all those adventures. If anything, that should have given him the confidence to really get angry and demand that they put every last object back or they get to taste the elf-sword!