I had to read this one when I heard about it. The Boxcar Children discover geocaching. I hadn't read one of these stories in years and it was indeed strange to hear the Boxcar Children discuss the Internet and using laptops.
When they're playing in the park their dog Watch discovers a box full of stuff. When they try to take it home to figure out what it is a father and son come along and tell them to leave it alone and that it is part of a game called geocaching. As the kids start to get into the game and meet other players in the area they, gasp, come across a mystery. Geocaches are dissapearing and it's up to them to find the thief. Needless to say, they succeed and all is well.
It's a decent introduction to geocaching for any kids in the 3rd or 4th grade.