Colleagues working on a site with a tree menu reported an issue that we've now tracked down to a bug. Certain nodes in the menu were not getting their "+/-" symbol to collapse the node, even though they did have pages beneath.
It turned out this happens if a) all the sub-pages have their view permissions set to only Admins and Content Admins and no other roles, and b) the logged in user is a Content Administrator but not an Administrator. If a full administrator logs in, the expand/collapse link appears no problem.
I've checked the html - this is not a styling issue, it really is the case that the markup for the expand/collapse link is not present.