General help

"The problem is rising when the same authenticated user tries to use the global search : he finds only the courses in which he is enrolled manualy and not the course with "guest enrolment" access."

Correct, Guest enrolment does not know who that user is at all, they are just the Guest user when inside such courses, as the real user is not enroled there.  

So, technically, users don't have access to such courses as themselves, which is how the Global search works now:

"If your search area indexes contents that belong to a course the user will only see results that belong to the courses where they have access"

https://docs.moodle.org/dev/Search_API#Access_control

You could put in a Tracker feature request for this.