Flower shops buy greenery from people for use in bouquets. I used to sell the florist I worked for clippings from the trees in my own yard. So it's possible that your trees are being harvested by a thief who is then selling your tree parts.
I suggest you take a picture of your trees, and maybe bring in a leaf sample to the different florists in your area and see if they are buying your tree parts, and maybe you can get them to stop it.
If massive amounts of clippings are disappearing, then you might even be losing out to a wholesale florist supply service.
I find it very unlikely that someone would regularly steal from your for their own personal arrangements.
I would not be surprised if illegal aliens are doing this.