Unless the car itself was unlocked, and it had one of those trunk opening buttons in the glove compartment. Kids might have been playing in the car and discovered the button.
My Camry has a trunk release down by the drivers seat. Mine is a 1999 and one of my sons still drives a 1992 model! Over 200,000 miles and still going. Anyway, it also has the trunk release by the drivers seat.
That is possible but I have only seen that button on newer models. I bought a new Buick last summer with that and have to remind myself to keep it locked since all it would take would be a smashed window to open the trunk.
How ironic to watch the deputy walk around the car knowing that the kids were in the trunk. And disturbing.