[jsb5717]

To me there are a couple options. One would be to remove the window from the opening to inspect the installation. Is there something in the way keeping the frame from laying flat to the wall? That would require new butyl tape and a sealant cap bead to properly reinstall. Second option would be to just, as you said, cap it with some Dicor. Either way, it shouldn't be too hard to prevent future leaks since you know where it's coming from.