What's the Name of That Book??? discussion

This topic is about
Someone Was Watching
SOLVED: Children's/YA
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SOLVED. YA Mystery published around the 1990's: Boy and his friend (middle school? almost high school age?) set out to find his missing baby sister (who was lost durring a family camping trip) after finding new clues that his parents won't listen to. [s]
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The little sister was lost after going on her own during a camping trip. after re-watching the last home video of his sister, the brother notices there is ice cream truck melodies playing in the background. he makes connections to the owner of the ice cream truck, and after no one would listen to him, he and his friend put their money together to set out and once and for all figure out what happened to his sister.
As it turns out the sister was in fact kidnapped by the woman who owns the truck, and I remember that the woman who kidnapped her would constantly keep the sister by her side, and dye her hair to make her unrecognizable.
Thank you so much for your help!