90s YA mystery/thriller with TSP recurring theme

I’m trying to remember a YA book I read in the 90s - I believe it was a mystery/thriller, but it could have been sci-fi as well. The only solid detail I remember is that there was something unexplainable or creepy going on, and the main character kept encountering the acronym TSP as part of the weirdness. For example, I remember the MC seeing a friend wearing a shirt with TSP on it, but it stood for The Space Project. Another example in the book was someone telling the MC recipe ingredients, and the MC freaked out when the recipe called for a tsp (teaspoon) of something. Not much to go on, I know, but hopefully this sounds familiar to someone out there!