Did they break in or was there no signs of forced entry? I wonder if they "bumped" the door lock to the exterior door to your garage? If you check out any of these videos online about "lock bumping," it looks trivial to do with inexpensive locksets. You can't even really call it "picking" the lock. I'm not sure what level you have to upgrade to to avoid it. Probably anything considered "commercial grade" should suffice. You don't need Medeco or locks with special keys to defeat lock bumping, from what I've read. Another alternative to the lock bumping is to add a dead bolt that only locks from the inside.

I saw a technique on TV not long ago that was so easy and I was surprised I had never heard of it before. If you have a garage door opener with one of those dangling cords for an emergency release, it's easy to use a coat hanger pushed through the top edge of the garage door to hook the cord and pull it, releasing the door and giving theives access to your garage. From the outside, a door stopper wedged into the top edge of the closed door opens a gap to push the coat hanger through.

Remember, it IS a safety feature, so if you have children or pets that could possibly get trapped under the door if the sensor malfunctions, you should think hard about this, but the counter measure the TV show presented is to use a zip tie to lock the release lever in place. An alternative might be to re-position the cord in such a way that you can't snag it--or maybe removing the little knob on the end of the cord so it can't be snagged with a hook. You could also conceivably mount a shield along the top edge of the door to prevent anything from being pushed through the door crack without interfering with the door operation.