I’ve been searching for this book for years. I cannot remember all the details but the basics:
Farm kids during the great depression, I think a boy and a girl. They go exploring a cave nearby and pretend its their mansion. So they don’t get lost they use breadcrumbs but birds eat it. They decide to use string instead. A mystery of some sort is solved. I seem to recall mention of the Hobos that would come around looking for work, and their family took one in and fed him a meal. At the end of the book he left markings on their fence that was code for “friendly family” or something like that. Other details: I THINK they took a canary into the cave, there was something to do with the underground railroad, and ration books were mentioned.
I read this book when I was in 6th grade, possibly 1977/78. It was an old book at the time, hardbound with the old fabric book covering. I think it had illustrations but can’t quite remember–I read a LOT in those days. This book belonged to my mom who was born in 38–sadly its been long lost.