Delivery envelopes do have a knack for getting underneath things and they're often gray and not readily visible. As Skinnylinny suggested, say a prayer to St. Anthony (always works for me too).
BTW Coolgran65.....Your post reminds me of a time when my middle child (an avid snooper and by then old enough to know there was no 'Santa'), began uncovering all my hidden presents before the holidays. The other two siblings got wind of it and very soon bribes were being exchanged to extract information. It wasn't long before DH got into it too.
The following year, I preemptively kept their presents locked up in my car, wrapped them during school/work hours and then stashed them right back inside the trunk (boot) of the car. I even kept my car keys with me, just in case!
Then my DH's family (travelling by plane) announced they were coming to stay with us the week of Christmas.
. Early on Christmas Eve, my BIL and SIL wanted to go Christmas shopping so I offered them my car and gave them the keys (not thinking clearly).
For the better part of the day, our Christmas tree was bare (no presents). When at the last minute, we were finally able to bring them out and put them underneath the tree, the look of relief on those dear children's faces was priceless!
A very thoughtful gift, Coolgran and one you're likely to find soon. Merry Christmas!