Definitely a problem with scissors in my house too. They either disappear from their normal place, or I hide them and forget where I put the - either way, the dog usually gets shouted at.
For me it's also 10mm spanners, of which I have loads but can't find them; and tiny flat-blade screwdrivers (used for all sorts of scraping and cleaning) but aren't ever there when I need them for their real purpose