When calling the function Gtk::TreeView::get_path_at_pos, gtkmm takes a copy of 
the returned GtkTreePath object, and passes this copy back to the caller of the 
function. The GTK+ docs specifiy that the returned GtkTreePath should be freed, 
but gtkmm just forgets about it.

Changing the line 

path = TreeModel::Path(pTreePath, true);

to

path = TreeModel::Path(pTreePath, false);

in gtkmm/gtk/src/treeview.ccg at line 259, so that you don't take a copy should 
fix this.