I think I've cracked the case! Derrick was walking into the coffee shop to find no one but a pair flip flops their! He picked the shoes up and looked at it! And it said "AA" so he took it! He knows that a pair of flip flops signed by Aunt Arctic (a famous mascot) would be a fortune! So he took the flip flops and ran off with it! When Rosie waked up from her nap time she found it missing!

Solved by Kricken!