Re: Our Linux IP CCTV
"How would I ever know the camera had been hijacked?"
This could be tricky. Packet sniffing (ie looking at the network traffic to/from it) might help, especially if your camera seems to be trying to access a jewellery website.
Without a fair bit of expertise though your best bet is to make sure that the firmware is as up to date as possible. Check online to see if there's any vulnerability reported for that particular camera (or whatever it's software is based on). Finally, limit the internet access of the camera to the absolute bare minimum necessary through your modem/router. If you don't need to access it over the internet, then remove all it's access.
"Is there code I could run?"
Not really, sorry.
"Would BT (sigh) be able to spot 'a signature' of naughtiness?"
They could, but they almost certainly wouldn't.
What's the make and model of the camera? People might be able to advise you if it's got know problems or not.