Guests leaving house unlocked on checkout

[Merridee0]

I have had a few guests recently who have left the back door closed, but unlocked when the leave (front door is locked though).
How do I stop this? 
My front door is a digital code lock, back door is key entry with a basic internal push button lock. I don’t give guests the back door key. 
I have an automated message set up to remind guests to lock both doors upon check out, but I’ve had 3 guests in the past 2 weeks who’ve not locked it, leaving it open all day until I arrive home after work.
Would you recommend a self locking door lock that locks on closing? 
TIA, Merridee

[Fred13]

Self locking may make sense. It appears this is very important to you, maybe have someone check that the guests did lock it. This eliminates a source of discontent with otherwise happy guests on their way out.

[Mike-And-Jane0]

@Merridee0 not giving guests the back door key and fitting a self closing auto locking door will guarantee people getting locked out. Can you not just remove the internal door knob mechanism and effectively make the back door operate the same as the front door?

[Merridee0]

Thanks for your reply.

Yes, guests getting locked out with an automatic back door lock when the door closes is my concern, which is why I wanted to ask this community what they do if they have guests leaving their property unlocked. 

Guests locked out in the backyard can easily walk around to the front door and open the front door via the digital lock if the back door closes and locks automatically.

My front door is a digital lock that has to be locked manually when guests are inside, and when they leave they have to manually press the “lock” button on the keypad on front door, which guests are doing. 

Worst case scenario: if guests go out the back door to my backyard, and the back door is accidentally closed and automatically locks behind them so they are locked out, AND they’ve locked the front door from the inside, AND they forget the 4 digit code to the front door AND they have left their phone (with my message giving them the code) inside.

In this scenario, the only way they could get back inside is to contact me (perhaps by going to a nearby public phone box) to get the code. My phone number is written in a prominent place near the front door. Is this an acceptable “risk” solution? 

Thanks, Merridee

[Mike-And-Jane0 · Accepted solution]

@Merridee0 that (worst case they call you) feels a better solution than a guest leaving the key inside and a self locking door closes on them which is a risk in many Airbnbs.