SSH passphrase dialog missing

I switched from an El Capitan machine to a new MacBook Air with Sierra.
With El Capitan, when I ssh into an host from terminal, the SSH passphrase GUI dialog showed up, and then it never asked again the passphrase for the whole login session.

Now with Sierra the dialog is not showing up, I recurred to adding the following lines to .bashrc:

eval $(ssh-agent)
ssh-add

but the drawback is that every new terminal asks for the passphrase.

How to diagnose the missing dialog?

Instead of ssh-add, use

ssh-add -A

This uses the Keychain to find the passphrase, and since your Keychain should be unlocked because you're logged in, you won't be prompted.

(Sierra drastically changed how ssh keys are handled by the OS. The dialog is no more.)