iPhone connected to home Wi-Fi when away

Silly question but when the notification appeared was the device in question at home or roaming? 

 Seriously, this is notification as result of security settings which is a good thing and primarily intended to notify users of unauthorized access.

If its a device that's familiar and or trusted just ignore it.

However, if desire is to dig deeper, check notification settings on whatever system, app, provided it. From there look at the timestamp of the actual activity... Majority of the time these type notifications are not real time and even when they are its usually due to VPN or other route the authentication information took that the home network which is keeping tabs on what it thinks is expected user activity and when deviated from the the notification is made. Further, just out of curiosity what was the Mac address and IP (which is dynamic) included in the message? If not familiar or unknown then potentially more investigation would be warranted. 

Just my .2 cents

 Thanks and good luck.

If you connected to a public network, it is possible that your phone was 'spoofed', and the one spoofing it connected to your home network in order to find the login credentials.  I suggest that you change the login password on your home network.  I would also suggest that you should never connect to open public networks; they are *NEVER* secure if they are open. 

I've noticed the Xfinity app update cause a delay in notification.  I was installing IoT devices and got no notification.  I left to get some food, but then got notifications saying new devices connected and they carry the name and MAC address of my IoT devices.  You might want to reconfirm if MAC address is really your phone or not.  Just note that 2.4GHz and 5GHz are supposed to have different MAC addresses.  Depend on manufacturer, but they are usually in sequence by not always on the same digit.  If all but 1 or 2 characters are the same, then it's probably your phone just connecting on different band.  It's possible while you are home, your iPhone connected using 5GHz (usually it prefers this) but when you leave, your phone try to stay connected and switches to 2.4GHz and if it never connected before, it may appear new to the router.  If the address is completely different, then it's possible  you have a neighbor that have sniffed out your iPhone's MAC and also your WiFi credential.  Change your credential is the best option.

Hey! Notifications can sometimes come in delayed from when a device connects to the In-Home Network. Have you ensured that no other devices have been connected such as a television, alarm clock, phone, smart watch or more was able to get connected? If you are not aware of what device was connected, you can always take advantage of the features we provide you within the xFi service such as Managing Profiles for your devices or Personalizing and Controlling Your Home Network with Xfinity xFi to pause devices from being able to access the Internet until you can identify which device was connected. If you need any further assistance, please do not hesitate to ask and we would be more than happy to help.