Eastern Sierra locals (myself included)can tell you that, over the years, there have in car break-ins at nearly all our trailheads. Many of these break-ins were done by locals, but sometimes by other hikers. I hope our visitors don't trust to luck or chance to protect their valuables.
Here is a good briefing on Whitney Portal. The section on bear break-ins is especially important:
http://timberlinetrails.net/WhitneyTrailhead.html
