We've been having this discussion at my house also. There have been a rash of ATV thefts in my area. I've got my quad chained up with log chain and a lock that would take a grinder (and likely an extra wheel) and my tack room as well but I'm still uneasy.
My brother in law is an avid hunter and he is bringing me a trail cam to test that runs on wifi (hubby is the cable guy so putting an extra modem in the barn is an easy fix - house wifi won't reach my barn unfortunately) and emails when motion is detected, can be set at different sensitivity levels - so it wouldn't go off for the barn cat but would for something larger and can be watched remotely through my pc or phone. They are relatively inexpensive, less than a hundred bucks a piece. I'm hoping it will be a cheap, simple answer. If not we are going to bite the bullet and have a security system installed.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.