I think that the theme of the book "Found" is there is always a bigger problem. I think this because at the beginning of the book Jonah was worried about trying out for the 7th grade basketball team, but when he received a mysterious letter all his priorities shifted to finding out who sent the letter to him and his best friend.
WARNING NEXT PART OF POST CONTAINS SPOILERS. DO NOT READ ON IF YOU ARE PLANNING ON READING THE BOOK "FOUND" LATER. YOU HAVE BEEN WARNED.
The second reason why I think that the theme is there is always a bigger problem is because for most of the book the team had thought that they were just kidnapped children from third world countries and that they were going to get in trouble for finding out where they came from and then they would probably be sent back to where they lived before they were kidnapped. But when they entered the cave they found out that they were the missing children from history and that they had to choose between going to the future and living perfect lives or going to the past and sacrificing themselves to stop the time ripple and stop time from collapsing in on itself. This shows you that every time you think that there is a big problem there is a bigger one right around the corner.