Can car trunks open by themselves?

I'd imagine it was possible. I know that back in the day, Ford only had 9 variations of keys for each model car they put out. Considering, that is a whole lot of cars that you could conceiveably open with your key. :wink:
It is possible there is some sort of interference that causes the trunks to be opened...I don't know though. Perhaps it is a haunted garage and this is a sign.

There is a famous/infamous problem with garage door openers that work for all the garages in the same neighbourhood.

The key to my old Honda Civic opened the doors to many other Civics and some Accords (don't ask how I found out ). Luckily, it didn't start the engines.

I wouldn't be at all surprised if auto trunk locks can be activated by controls from other cars. Annoyed that the auto-makers can't be bothered to do anything about it, but not surprised.

Have a relative who had a friend come to visit, told them they could borrow their car (Honda). The friend drove all over town in that car while the owner was at work (took the bus). That night, when they went back out to the parking garage, the friend went to the car she'd been driving all day - turned out it was someone else's car. True story.

Funny you mention this because on my way to work this AM, a BMW 3-series on was parked on my street with the trunk open and no one around.

That's awesome cjhsa! You mean not only the electronic door unlock, but the key to the ignition worked!?! It does worry me. I have a black Honda and there are so many exactly like it. I just never heard of these situations and I probably would not have noticed so much except for that one time when I came back to my car with the trunk slightly open. I knew I did not open it at all and thought perhaps I hit the trunk release accidentally. Then days later I began to notice other cars every once in a while with their trunks ajar. I thought maybe some one was breaking into them, but never saw any notices or anything, which I would have expected if there were thefts in the area.

This was awhile ago, before Honda's had electronic locks. Honda had a slogan, which still applies today in many ways - "We make it simple".