Unfortunately, this has happened to me before! I was trying to get the train station upgrade, so I had lots of people passing through my town all day. It didn't even occur to me to keep an eye on people. All of my blue roses and Jacob's ladders went missing, a few bags of Bells that I had sitting near ReTail, and some of the rarer mushrooms. It's made me really weird about having other people in my town!
Wut???? That make me wonder back how I able to upgrade my train station last time with the 100 people cross by requirement O.O thanks god no such thing happen to me...probably I am too poor to have any valuable thing lay on the ground that time lol