I’d be getting myself a cheap wireless garage alarm…..
Similar thing happened to me during the summer. Garage door was open. I was about to go out for a ride & was in the back of the garage with no light on fart-arsing about. Looked up to see two scrotes walking up the drive. I reckon they’d seen the door open, but not me ferreting away in the back so thought they’d have a quick look see. I walked as quick as I could out of the garage to put some space between me & the bikes, to hopefully keep them in the dark.
The two scrotes on seeing someone walking out of the garage looked initially shocked, but to their credit they quickly recovered & said they were knocking on doors asking if anyone wanted any odd-jobs doing. I told them I didn’t, and they wandered off.
I then went for a ride fully expecting my garage door to be wrenched open upon returning, but it wasn’t.