Having something rammed down your neck like you're a 10 year old never really helps. Being a victim helps people remember the basic things. Have a couple of people walking around placing stickers on laptops that are unattended. When the owner returns they'll see a sticker saying something along the lines of "your laptop has been stolen", and they'll quickly feel foolish for dropping their guard.
That sticker solution is much more '10 year old' than a general warning to watch out for your own stuff. It gamifies the situation, rather than being direct about it.
Security is always an individual's concern, and to the extent one does not think about it just means they are unconcerned. Are bad neighborhoods supposed to have warning signs? Or maybe city-wide address systems should be used to warn people of increased crime every time it gets dark?
When you have 1000 students in a hackathon, then yeah it's a good idea to point out the difference.