Are you sure it was flashing Yellow ?
According to VM, here is the breakdown of all the lights and what they mean on a Hub 4 and it does not mention yellow lights.
When you first turn the Virgin Media Hub on, it will show a solid white light for around 60 seconds; it is also the white power light of your Virgin Media Hub.
A red light means an issue with your connections even though the Hub is on. As you connect the power supply to the wall and the Hub, make sure it is tightly connected. You need to verify that the white coax cable from Virgin Media’s wall sockets is also tightly connected to the Hub.
The Hub is on if you see a dimmed white light, which means that everything is okay and that it is usually working; there is no need to do anything or call support. A dim white light on your Virgin Media Hub is good.
This means the Hub is in modem mode and won’t be sharing WiFi. If you’ve put the Hub into modem mode yourself, you’ll probably be using your own router alongside the Hub.
You can take the Hub back out of modem mode manually, or you can do it by resetting the Hub – just press the pinhole factory reset button for 10 seconds with the tip of a pen and leave it for 5–10 minutes to start up. When it’s finished, you’ll see the light turn white.
If the Hub flashes green slowly it means it’s downloading a software update. The Hub will take a few minutes to reboot once it’s finished downloading it.
This means that the Hub is on. When you press the WPS button on the back of the Hub, the light ring will flash blue gently for 2 minutes. This is your VirginMedia router’s way of showing you that a connection is being attempted.
When the WPS connection is successful on your Virgin Media router, the light ring will stay blue for 20 seconds. If the light ring flashes rapidly, it indicates that the WPS connection was unsuccessful, and you should try again.
This means that the Virgin Media Hub is on. It is booting up and should be ready in 5 minutes.
