Auth User Token still works after logout

* Operating System version: OSX 10.12.6
* Firebase SDK version: firebase 4.3.0
* Firebase Product: auth

### Describe the problem

When I reload my Angular-4-based webpage, I reacquire the Firebase user token, via:
```
firebase.auth().onIdTokenChanged( user => {
if (user) { user.getIdToken().then( (token:string) => {
...save token...
})
})
```
and I have access to my password-protected Firebase database.
However, this still works after I have logged out, via:
```
firebase.auth().signOut();
```

How can this happen? How should I be logging out if that is not correct? How do I get a status back from signOut()?