FWIW, I've always found that name of "Topic marker" to be confusing. It helps me to think of it as an implied contrast, or else as being the same as where I could use "As for" in English. It can come in places where you would have seen 를 or 을 when there is an implied contrast or comparison in that place even.

One way it can happen is with contrasts such as you will find with compound sentences separated with 는데.