Read-only smart lock

You've surely got a loved one (or maybe you are that loved one) who is always worried about having left the door open and has a minutes-long compulsive ceremony of checking the lock several times... only to turn around mid-trip to check one more time.

It might be comforting to be able to check if the door is locked at any given time, perhaps it would lower anxiety and facilitate arriving at work and social functions on time.

Smart locks still have a lot to prove on the security front, so adding new attack methods to your door through the internet might not help with anxiety that much.

The nice middle ground might be a read-only smart lock that's only capable of reporting if the door is locked or not without having any mechanism to open it. Sure, if it's connected to the internet, it can be compromised, but at least it could not be used to open your door over the internet without your consent. And it's a simpler system to lock down properly.

The fine people of the Internet of Things Stack Exchange have discussed this as a DIY solution: it's good inspiration.