This is another idea that I'm posting here so I can point out that I thought of it first if someone else thinks of it later.
It's a thin, relatively flat object with pressure and light sensors on it. When you bring your laptop to a public place (like a cafe or a restaurant) to take advantage of the free wifi, you place this underneath it. If anyone tries to pick up the laptop without properly disabling the device first (probably via a passcode), it emits a loud noise alerting everyone nearby that a laptop theft is in process. Thus, you can safely get up from your seat and go up to the counter or visit the restroom without worrying about someone swiping your laptop and sneaking off with it unnoticed.