I looked this up on Google and got a ton of hits. Sometimes this will happen mysteriously for many cars in a region then stop. (I know that's not the problem here but it's intriguing.)
For example, from a Las Vegas paper: Valley has keyless encounters of the weird kind
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According to the Federal Communications Commission, the low-power radio frequency transmitters inside keyless entry devices are similar to those found in other everyday items such as garage door openers, remote-controlled toys, cordless telephones, building alarm systems and the rapidly spreading wireless fidelity computer networks, which are commonly referred to as "wi-fi."
Paul Oei, an electronics engineer with the Los Angeles office of the FCC, said keyless entry systems operate on unlicensed frequencies. The devices can fail when they are near an antenna emitting high radio frequency energy. But that scenario would affect only vehicles in a limited area, he said.
There has been a rumor going around in one of those dreadful e-mails that your friends and co-workers feel compelled to forward to you all the time. If you lock your keys in your car and you have a remote keyless entry system, you can get outside help to open the car if you have your cell phone with you. Just call someone that has a duplicate key fob that will open your car. Then, hold you cell phone near the door lock and have the person with the key fob call you back. The person with the key fob should then put the key near their phone and push the unlock button. The door should open.
I was skeptical, to say the least, about this rumor, and was about to dismiss it as one more Internet hoax. But I thought I better try it out first. Well, low and behold, it works. I tried it with both GSM and cdma cell phones, and it reliably opens (and relocks) the car.