I've taken a peek into approximately 10 GWT Accounts and found just 1 with Soft 404s. I'm guessing that the Soft 404s link does not appear if you don't have them, I'm almost certain of that.
The 1 site I did find with the Soft 404s is one we consulted on a few years ago. We provided specific instructions on what they should do with 301, 302, 304, 404 and 410. They did not follow those instructions and now have the Soft 404s showing. Message for that client: If you're reading this, I TOLD YOU SO! ;)
If you have flaccid 404s and you think Google is incorrectly reporting, I'd double, triple check everything. If you have any sort of redirect chain in your 404 handling, I think that may present some challenges. I dunno, not my forte, I don't do redirect chains, I think they're poisonous. :)