This is book number 113 in the The Boxcar Children Mysteries series.
One day in the park, the Aldens’ dog, Watch, comes across a mysterious box with a note inside. “Congratulations!” the note says. “You’ve found it!” The box is part of a game called Geocaching. Soon, Henry, Jessie, Violet, and Benny are searching all over town for hidden boxes and making friends with other kids in the geocaching club. But is one of the club members a thief?