Grannyactivist, I have lost count of the things that go missing, and we regularly have a row discussion about him taking things without asking, because at times I have thought I was going mad. He is a teacher and he has to give lectures occasionally, as part of assembly, that have a moral value. (not about stealing, I presume) He takes props. No-one else takes props in, just him.
So far this year alone, I have gone outside with my washing to hang it up and not been able to find the rotary drier, gone to switch on the standard lamp and it had become part of Narnia, all of my baking equipment went missing and two days ago my lovely wrought iron tall candle holder that fits in my fireplace had gone. I was livid. I was even more livid when he came home and simply plonked it down on the rug and left it there. I made him return, pick it up and put it back exactly where he had filched it from. I think he thought I was joking at first, but when I think how cross he would get if I touched anything in his shed without replacing it I think my annoyance is justified. I get so worried that the kids will break my things ( a besom broom that came back a different shape springs to mind) or that he will forget to bring them home, which is what happened to the baking things for about four days. I don't think your annoyance about your mouse is petty at all - he thinks nothing of taking my phone charger if he can't find his.