The reason I say this is that at lunchtime, I drove to the bank, locked the car with the remote as usual, and went inside to make a deposit. Five minutes later, I came out, pushed the unlock button, and nothing happened. Twice.
I knew what was coming next. There is only one part I still remember from the speech I was given when I first got the car. Over the last year, I have often wished it was the part about how to check the oil levels, but that escaped me completely. Instead, I remembered that if I needed to use the emergency key, or if my remote stopped working, I could disable the security system after the alarm stopped and get the car running, and the instructions on how to do this are in the owner's manual.
I prepared myself as best I could, unlocked my car, and then stood around reading the owner's manual and looking embarassed for two minutes while my car alarm went off and annoyed the entire area. Finally it stopped. Step two: turn the ignition to Run and back to Lock. Not a problem. Step three: Wait 15 minutes. Well, I had a book... After five minutes, I was getting warm, so I opened the car door. And the alarm started again. I shut it and it stopped. Tried the ignition, just in case 15 minutes was a generous estimate. The alarm went off until I turned it off.
I grabbed my purse and left (as quickly as possible, since opening the door set it off again) and went to apologize to everyone at the bank. Then I walked up to the post office to do the other office errand, and by the time I got back to my car, it had in fact turned off the security system and could be used.
No real harm done, except for the embarrassment, since the car alarm is hardly an essential system. Presumably what happened is that the battery in the remote died. The book says their average life is two years under normal use, and as far as I know it's never been changed in the 7+ years I've had the car, so that should be a safe guess. It certainly shouldn't be the receiver in the car, which was only replaced a few months ago. I'm going to try to find a replacement battery, which should fix it. If I can't find one or it doesn't do the trick, I think I'll just do without.
ETA: a quick search says the battery in question is $3.49 at Radio Shack.