From Destini:
This has been bugging me off and on for a while, and for some reason I just never asked it. Can anyone help? I'm sure that I just missed it...
Here is an idea about how someone could know if their door was locked on or around a specific day 3 years ago. I might be able to remember this if there was a period of time that I habitually did lock my door, then for some reason gave that up (or vice versa).
For example, say that I lived with a roommate, and we were pretty diligent about locking our doors even though it was just out of an abundance of caution (meaning that we did it out of habit or something even though we never had serious concerns about intruders). However, the lock starts sticking for some unknown reason, we call the maintenance or whomever a couple times and they fix it but then it gets sticky again. (Based on a real pain-in-the-butt lock on one of the doors to a previous lab.) We are tired of the inconvenience and we don't really have serious concerns about intruders, so we just give up locking at some point. (Note, I would not personally do this - I would have the lock replaced - but I bet that not everyone would.)
Or say that my roommate and I are living together, not really worried about intruders and therefore not very consistent about locking up. Maybe we lock the doors when we are both in "for the night" or when we leave the house for class in the a.m. even though we are not on exactly the same schedule. But on Mondays-Wed-Fri I get out of class at 2 and come home to run before I study, so just while I'm out running I leave the door unlocked so that I don't have to carry the key with me. (This is kind of common practice among med students at my school, anyway.) But we start hearing stories about other apartments in our general area being burglarized, and at some point decide that it is not worth the risk to leave the place unlocked and unattended even if it is only for an hour.
You can be the judge on how realistic those particular situations or the ideas in general may be.
Just some thoughts that occurred to me, because sometimes I do wonder, "How the heck would I know?!" when I hear detectives' questions (in real life or on tv, lol).