Marnie Anderson, your not alone. We too had flowers stolen.
My grandfather's flowers were stolen from his grave after I spent hours cleaning pine needles up and cleaning the grave markers and spending several dollars to buy the beautiful flower arrangements for our loved ones only to find someone liked his flowers.
In years past they chose to take my grandmothers several years in a row. Not only that, my husband used to make beautiful crosses for my son's godfather and yes, all of a sudden for three years in a row they were stolen too, so we had to quit making those. Just remember they will have to answer to one person someday and I'm sure the person you put my grandfather's flowers on are looking down on you asking why.