Nah, not stupid—it's only obvious after the answer is shown.
If it helps for next time a problem arises like this, here's my thinking process for this question: the findGrep() function is a method of what objects? various text objects, textFrames, pages, spreads, documents all have it... all no good for your situation unless we iterate over all the open documents. Hmmm, could do that, combining the results after each findGrep. But then I realised there was an object that "contains" all documents: App. I had a look in the documentation and yes! it has a findGrep method. Tested your function and it worked.