Thanks, Miruna. If there's no external HTTP sites referenced for images, are there any other possibilities? 

Someone at the customer site with experience with this message chimed in with this below, but I'm not sure what to make of it since no external images are being referenced in the form, and I don't understand how to get it to automatically rewrite. 

For web applications to remediate mixed content errors, the two methods that I'm aware of are:

1.        Load all resources via HTML source instead of specifying the HTTP or HTTPS protocols. For example:

Use “//domain.com/path/to.file” instead of “http://domain.com/path/to.file”

1.       OR, the CMS or application can use a plug-in to automatically rewrite HTTP resources to HTTPS

Right, those are the generic mitigation steps for mixed content warnings. But in order to rewrite paths, you need to find them.

If you look in your browser's developer tools when you load the page, it should show something like this:

That should point you in the right direction for finding the culprit.

The screenshot above is from Chrome. IE is not as obvious about it: you get a message in the console tab, but you have to hunt down the file on the Network tab

Thank you. This is very helpful. I checked the Console and Network tabs and I did not see a mixed content warning despite getting it at the bottom of the page.
On the network tab, everything that is listed there has HTTPS and there are no HTTP instances listed. So I'm not sure at this point what could be causing this message. Also, this is only being displayed when a user clicks on a task in the inbox and the form opens for approval.

So that's where you'd want to look at the calls in the developer tools. The form might be using a JavaScript file or something like that referenced directly from another website.

I checked the java script and there's no references to http. Please see screen shot. Also, a co-worker says that she gets this message just from going to the forms inbox, but this message only appears for me when I open a form to approve.