I know this is not DIY, but I recently had this problem and it cost me $130 to solve it.
I use the Asante Smartbot camera connected via WiFi. I easily set it up to email any suspicious activity to my GMail account.
This is a WIndows solution (Gasp!)
I live between Mt. Adams and Mt Hood. If you look around the area there are many extensive lava beds and lava tubes. The walls of the Columbia Gorge consist of multiple 50 ft. thick lava layers - just saying. IANAG