https://github.com/Slicer/Slicer/pull/83/files

Please add a test.
A linear search could get slow when looking for a markup near the end of the list. The ids are generally numbered by index, could you add in a "base case" that determines an index from the ID number and checks at that index and maybe +/-1 around it for the matching id?

Is there any mechanism for organizing Markups ? (Like move up, move down).
Because if there is such mechanism, that means maybe a markup with and ID of 22 could be in index 5 if it has been moved.

Yes, Markups can be moved down/up or removed and that will shift things, but a quick check at the ID index will generally succeed.

Should I do a double check ?
Check if +/- 1 around the index in the markup ID is the right one, if not found, do a check of all markups in the list until finding it (or not) ?

yes, that's what I was suggesting.

Ok, I can implement that.
However, I'm not sure what to do for the test.

Thinking more about it, +/-1 around the id index can be skipped, that might be too much code for the cases where only 1 fid was removed or moved.

Add a few calls to the vtkMRMLMarkupsNodeTest2.cxx file to check that the index and id match up (also add tests for null and unmoved/deleted markups in a list).

Is this test enough ?
https://github.com/lchauvin/Slicer/commit/120bf1dc95790245b70f6a0142ee90db0ae25d62

The test is a good start, I'd also add testing the failure cases (trying to get a markup with an id that doesn't exist in the list, trying with a null ID).

After looking at the code, you don't need to add the test around the index in the ID, it won't be very helpful.

vtkMRMLMarkupsNode::GetMarkupIndexByID() method is now available that implements this functionality(http://apidocs.slicer.org/master/classvtkMRMLMarkupsNode.html#a29ef78f692c11028e869be840791afc1)