golf mom, Talk to a security company about your situation and have some cameras installed. You can also purchase them online and have your son install them, but I would suggest that you not only put one outside, but put two inside the garage, one filming the front at the doors and if you have an entrance door from the garage one installed to film at that door.
As for the handle, it most likely didn't come loose. It's very easy to twist those handles until the unlock. Also, if they manage to twist them enough, they can actually use a screw driver to pry them more so to get in. My sister experienced this situation over the summer when someone got into her garage and stole her riding lawn mower while she was away on vacation. Those handles are only made to keep the honest people out. Also, do you have garage door openers installed? If not, consider getting them installed. He won't be able to raise the door if you have those and you can go one step further and cut the power to them if you are going to be gone for a length time.
Please, please be careful. Learn to be more vigilant. Check your car before you leave places that he knows you go. Walk around your home and see if things have been walked on near the windows. I don't want to alarm you, but I've had many things happen around my home and vehicle when my xh was nutty as a fruitcake. It's better to be cautious and know you are safe.
Also, get yourself some pepper spray and keep it handy. Also, take some time and take photos of the inside of your home and garage from all angles. That way if something should happen or things are taken, you have proof of what they items where that walked. Put these photos in a safe deposit box or leave them w/someone who will keep them safe.
I honestly don't trust your h right now. Please be careful.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.
