Rails flash notice won't go away in Safari?

I have a Rails 3.2 app that manages students. It has a fairly typical nav bar across the top (Foundation 5) which contains a quick search field. The nav bar is displayed on every page of the site.

If you enter a valid (numeric) student ID into the search field, you simply jump to that student's page. If you enter text or other non-numeric input, you get a flash error asking for valid input. If you enter an id that's not found, you get a flash notice saying it wasn't found. In either of the latter two cases, the controller should just drop you back to whatever page you came from and display the appropriate flash message.

For starters, here's the search field in the view:

<%= form_tag search_students_path, method: 'get' do %>
<div id="nav-search" class="row collapse">
  <div id="nav-search-field" class="small-21 columns">
    <%= text_field_tag :search, nil, autocomplete: 'off' %>
  </div>
  <div id="nav-search-icon" class="small-3 columns">
    <%= submit_tag '&#xf002;'.html_safe, class: 'button fa fa-search spin', name: 'submit' %>
  </div>
</div>
<% end %>

And here's the controller action:

def search
  session[:return_to] ||= request.referer
  if params[:search].to_i.zero?
    flash[:error] = %Q[<i class="fa fa-times fa-fw"></i> Please enter a numeric student ID.].html_safe
    redirect_to session.delete(:return_to)
  else
    id = params[:search].to_i.abs
    @student = Student.search(id).first
    if @student
      redirect_to @student
    else
      flash[:caution] = %Q[<i class="fa fa-warning fa-fw"></i> Sorry, we couldn't find a student with ID #{id}.].html_safe
      redirect_to session.delete(:return_to)
    end
  end
end

Lastly, here's the code for rendering flash messages in application.html.erb:

<% flash.each do |key, value| %>
  <div data-alert class="alert-box cbc-<%= key %>">
    <%= value %>
    <a href="#" class="close">&times;</a>
  </div>
<% end %>

In Chrome and FireFox this works exactly as expected. The flash appears for one request, then disappears. However, in Safari, once the flash comes up it never goes away for that page. So if you get the error flash on the home page, for example, you can refresh all you want. It stays put. You can go to another page, and then come back, and it's still there. The same is true for other pages. Once the flash message has appeared on a given page, it doesn't go away.

Thus my question: how can I get Safari to clear the flash after the first request?

I'm aware of the whole "flash vs. flash.now" issue when rendering pages. But even then, the flash will disappear if you simply refresh. I actually tried flash.now in this case, but then the flash isn't displayed at all in any browser.

Since this appears to be a browser-specific problem, here are some further stats on my system:

Mac OS X 10.9
Safari 7.0
Rails 3.2.16

One final observation. After playing around with this issue in Safari, I noticed that if I clicked my bookmark for http://localhost:3000/, that would clear the flash. Of course, all the navigation links in my site layout contain relative paths, whereas the bookmark is calling a full url.

Anyway, hope that made sense. Thanks in advance for your help!