I thought you were onto something

我以为你了解一些事情
onto在口语中有知道，了解的意思

不对：I thought you were onto something = I thought you were planning to do something = 我以为你们在准备/打算做某事呢
注意: let somebody in on = 才是口语里的知道，了解的意思呢 (:

应该是I thought you were upto something。