My project vanished

My Sim kid (Chase) brought home a project from school - a Medieval castle. I told him to put it down outside because the house is small and it wouldn't fit indoors. He did this and while I was checking on the toddlers, the project vanished. I checked my Sims' inventories and it had teleported into his aunt Jupiter's inventory. So I told her to put it down in the world and she put it by the grape bushes. When I looked again, it had vanished again.

I checked all my Sims' inventories and the family inventory and I've looked all over the house...no project. Has this happened to anybody else?