Just found another possible reason whereby no warnings are given. A user has created a template, and created a txt only template too. The footer had unsub in, neither of the above had opt out. I added opt-out to HTML but of course Plain Text was not expanded so i didn't know it was even in use!! And so setting the token in there fixed it. but at no point of the OptOut button being missing from either did i get a warning, until i saved, closed, continued and bingo Plain Text was expanded and showed the warning
I found a couple more situations that the greyed out button can appear which may be less than obvious
if there is no Resubscribe Msg set on the Response tab (no alert is currently provided
if you put in a Subject, but then decide to switch to using a Template the Subject gets changed to the Subject of the Template, so if the Template does not have one you may get surprised