Hi @max89,
Thank you for your question about caulking windows.
In internal applications where no water is present, No More Gaps Interior Multipurpose Gap Filler is usually my go-to product.
It looks like there may already be caulk there, but some slight movement has opened up a gap. If you wanted to fill it, the No More Gaps would be fine over the top.
You might like to check out How To Silicone a Gap for an example of applying a filler product such as No More Gaps.
Let me know if you have any further questions.
Jacob
