Re: API call or this-> function to find the current course section?

This really depends where you are. If you are on the course main page you will have section parameter (or not) indicating you where you are:

$sectionno = optional_param('section', 0, PARAM_INT);

If you are in some activity you should get course_module data and read the value of section field.

$cm = get_coursemodule_from_id('<moduletype>', $id);

$sectionno = $cm->section;

and how to know where you are? check the current page URL.

global $ME;

// activity

stripos('/mod/', $ME) > 0;

// course

stripos('/course/view.php', $ME) > 0;

 etc.