S., I'm no psychologist - but I did want to write and point out a couple things.
1st - let me say that a couple weeks ago, a close friend of mine who parked just RIGHT IN FRONT of a store in a busy plaza returned to her car to find a broken window. Her "extra set" of keys that she hides in her car were gone, and so was her wallet. (which she didn't have with her because she wasn't there to shop, but to take care of something unrelated). It didn't end there. When she drove home, her cat was outside screaming, her front door was wide open, and every single room in her home was trashed, and everything flipped over and broken...everything.. AND they maxed out her credit cards.. all in 1 afternoon. (This is why, in a separate post, I put that I've lost my weight watchers buddy).
--I think that it's GREAT that your daughter is going through this 'mental checklist' in her head. Because many people believe it happens to 'every one else.'
---You mentioned that she used to be tough and nothing bothered her. Either she's seen something on t.v., read about something in the paper (you know what's in the paper these days!), or a friend has told her something? OR - maybe, you were right about her sudden insecurities-maybe she sees this as a way to make sure things are "still in order, and under control."
I personally wouldn't discourage it. Sometimes I go to bed at night, and BOTH the main lock and the chain lock were left unlocked, and I wonder what the heck is wrong with my family because if I were the last one in the door, they'd be locked!
My 8 year old daughter is really good about this during the day, though, she'd have the chain locked in addition to the main lock, and my husband would come home from work just going nuts because he can't get past the chain after he's unlocked the door.. LOL
I would allow her to keep a small checklist, maybe on a dry erase board so she could erase it the next morning. "Did we lock the car on our way in? Yes, check! .. Did we lock the doors behind us? Yes, check! Did someone remember to close the garage door? Great, check!" and once she sees this mini-list has all it's check marks, we could put our minds at ease and get ready for bed!
----I'm not sure about beliefs, at our home, we'd choose to wrap it up in one little bedtime prayer together and pray that we are kept safe and sound. That's what I usually do with my daughter when she barges in our room when a bad dream had woken her up - we'd pray a small prayer together, and she always goes right back to bed feeling better.
Good luck! I hope I haven't overstepped any boundaries!!