When’s the right time to use a lock icon on a button?

A little background on the question: I've recently come across an instance where a user is submitting a form to check their status on something. I've used lock icons on buttons for shopping purchase screens (check-out), but in this case the user isn't actually purchasing something.

Curious what everyone's thoughts are on the right time to use a lock icon to show something is secure? Especially when that something is form with personal information vs. a transaction like checking out with an online purchase.

Here's a more visual sample: