(no pun intended)
It's a WWYD? I'm posting for traffic. sorry.
Short version ... we live together and she keeps going out and leaving the door/s unlocked.
The long version with detail and a bit of insight is this ... We all live together and have done for 5 years. She has her own living room with a conservatory attached and a little courtyard garden directly outside it which she potters in daily. DM is 84 and very doddery but likes to walk out round the village a bit (with her sticks) most days. She still seems as sharp as a tack. Uses internet. Checks bank statements ect. Forgets nothing else important.
When we moved into this house the front and back doors were those UPVC ones with a handle. Always open unless you actively lock it. Conservatory the same.
DM resisted locking the front door - preferring people she'd called over to be able to just walk in so she didn't have to get up and let them in. I hated this - one day i came downstairs in my bra and pants and the bloody computer repair bloke was marching into the house. ''Oh your mother said just walk in! ho ho'' 
I asked her loads of times to keep the key turned but she wouldn't. DM cannot hear anyone coming into the house from her living room, so it was a big security issue if the rest of us were out. (5 adults one child) DD4 was only 3 when we moved in. Didn't like door left open/unlocked for this reason also, plus pets.
So after 18 months of tussling with her about this we changed the door (at our own expense) to one with a yale lock. (£££ solid door) It has an easy handle on the inside, but you need a key to come into the house. When it's shut it's SHUT. (We also changed the back door.)
DM rallied hard against the new front door. First of all she just kept leaving the bloody thing ajar. To let people walk in (again!) and also when she went out (even if we were all out!) to avoid bothering taking a key. We worked through it all patiently. Extra pull handle on inside. Special graphite powder to make sure the key slips easily. DH made her a wooden tool thing so she could manage the key from outside (which hasn't been touched since). We are now at the stage where 90% of the time she uses the front door properly. Happy end? NO.
3 times in the last few months she's gone out and left the conservatory unlocked! Expensive secure front door - but anyone could walk in through the bloomin' conservatory!
The first time; i mentioned it and she said oooh did i? I've never done that before ect. Second time we said if she leaves it unlocked again we'll have to keep the key and lock it/unlock it ourselves when we're at home, but she was furious with that idea! Shouted and hollered and said we ''couldn't do that'' because she wouldn't be in control of when she went out into the courtyard and insisted it wouldn't happen again.
Well she's done it again today :(
DH will be furious. I haven't told him yet. I don't know how to handle it.
The big long story is because a big part of me feels this is bloody minded/can't be arsedness rather than a loosing marbles situation. But the upshot is the same either way - the house isn't secure when she does this.
WWYD?