Is it possible that they could have been stolen out of your mailbox? This actually happened to my season tickets one year, I figured it out because a family I knew through a friend ended up buying them from the SOB that dug through my mail. Our ticket office told me that this is actually not uncommon for folks who live in apartments or in more populated areas (like uptown New Orleans) to have their tickets stolen.
Package thieves are usually more active right before holidays, not sure if that's a problem in whatever town you live in though.