Why would a revised manuscript following very minor revisions appear as "under review" in the journal management system? I have a paper on an Elsevier journal with "accept, minor revision". There were 3-4 little modifications required by referees. Then, I have made all the necessary modifications and resubmitted. 

The status was "With Editor" for three weeks, then it changed to "Under review". What I have heard was that minor revision does not require another round of revision in most of the cases. In this case, does it mean that the paper is sent back to referees again? Or is it just with the editor who evaluates the last version of the paper?
It is not uncommon after minor revisions that the paper is indeed send back to (usually the same) reviewers. In most cases another round of revisions is not needed and they will simply accept the paper, but this is of course not guaranteed. 

For some journals, as a reviewer, you can indicate whether you want to see the paper again after minor revisions, I do not know whether this is an option for Elsevier journals.