French windows should have one pair of key mounted mortice security bolts or key operated surface mounted bolts.
If you're sure there's no-one left inside, don't forget to lock the mortice deadbolt.
Your door should also be fitted with a 5 lever mortice deadlock.
Side doors in detached garages should be fitted with a quality mortice lock to BS 3621 standard.
Thinner doors will be weakened by fitting mortice locks, so robust deadlocking rim locks should be used.