I've had thieves pry into the trunk area of my hatchback with a crowbar, even when the stereo was pulled out of the dash. They got the rear speakers and amp that way.

There are also thieves who will "case" a parking lot, watching who's going in and estimating the amount of time they will spend inside. They watch you store that thing in the trunk and it's all over.

You get lax about security, that's when they'll get ya...