It's generally a bad idea to mutate a passed arg but your implementation here will mutate `message` if there is already a period at the end of the message.

Also, I'm not sure the "RSpec could not determine which call generated this warning" bit has any value -- to a user, being told "I can't tell you any more about this" isn't useful or helpful.  There's nothing actionable for the user based on whether or not rspec can add a call site.  It would be a bit like if grep added a "can't find any more matches" message after everytime you ran it.

If there's no example, I don't think it should append anything.