I have had this happen a couple of times.
The last time they left no card so I didn't know anything had arrived. It was in the recycling bin (cardboard and stuff). We only found it because DH had gone out to put some stuff in the recycling and just happened to glance down before he did so. Just as well.
Bins have to be just about the most twatty places to deliver a potentially valuable parcel to. It could be collection day!!