'96 Honda Civic 4-door lock problem?

If I am reading you correctly, you are saying that the car is locked when you leave it, and unlocked when you return? The likely cause here is that someone who parks near you has a lock transmitter frequency very close to yours, and when they lock or unlock theirs, it triggers yours, too. Believe it or not, there really are not very many individual frequencies for power door locks, and in a very congested area, this is not unusual. The good news is that most systems are "scrolling", meaning that every time they are triggered, the accepted code will change a bit to prevent someone from being able to lock in on your specific frequency. You may be able to get around this by either going thru the relearn procedure for your remote key fobs, or by replacing them thru your dealer. When a system is triggered, it will accept the next signal give or take a few positions either up or down from the last one. This is so if you were to trigger your remote by accident a few times, your car would not refuse the code. So there is a "window" for each system, the current code plus or minus a few degrees. Try reprogramming your remotes. It is likely coincidence, someone who parks near you is on almost the same frequency.