Node tagged with more than one taxonomy, how to keep expanded the specific taxonomy menu the user has used to reach a node
I've done this in my modules hook_init() implementation by using sessions, menu_get_object and menu_tree_set_path(). Here's some example code showing my general approach:

function mymodule_init() {
  // Save the active path for users when on taxonomy term pages.
  if (arg(0) == 'taxonomy' &amp;&amp; arg(1) == 'term' &amp;&amp; arg(2)) {
    $_SESSION['mymodule_path'] = $_GET['q'];
  }
  // The user is on a node page.
  if (arg(0) == 'node' &amp;&amp; arg(1)) {
    $node = menu_get_object();
    // Assuming all the nodes in the taxonomy menu are of the same specific type.
    if ($node-&gt;type == 'mynodetype') {
      if (isset($_SESSION['mymodule_path'])) {
        menu_tree_set_path('menu-mymenu', $_SESSION['mymodule_path']);
      }
    }
  }