And now I have learned that just because there are perfectly valid reasons for the annoying character-trait it doesn't make it less annoying.
The book is set in the 50s and the MC has only recently returned to England after having been a charity-worker in remote villages in different part of Asia. As in: places where nobody locked their doors because there were no locks. She also doesn't care for her worldly possessions very much because for a large part of her life she had to do with very little.
Now as a result, she frequently forgets to lock her doors (of her car, her home, the shop she works in...) even though she now lives again in a place with door-locks (and where it's advisable to do so...just look at the murder-rates in picturesque fictional villages in cozy crime-novels). So when a body turns up in her shop she infuriates the inspector by being unable to tell if she locked the door the day before and she can't tell if anything is missing from her home because she has no clue what she owns. Well...and it really makes sense for her to be like this (unlike a couple of other cozy-heroines that are probably supposed to be adorably chaotic but rather make you wonder how they manage to find their way from the bed to the bedroom-door without falling over twice and putting their pants on their head because they're just so *~*adorably confused*~* all the time...) but...that doesn't change the fact that I sigh in sympathy with the poor inspector when she isn't able to give him a clear answer.
And it doesn't help that it's repeated over and over again that she keeps forgetting to lock her doors. It's almost the first thing we learn about her and then we get hit over the head with it over and over again.