Hidden as in you can't see it, not hidden as in you don't know it's there. Everyone knows phones have front cameras. And if someone could get access to that, they could watch you just as easily through a not hidden front camera...
The obsession boring people have with thinking that someone is going to hack their webcam and record some sort of salacious content of them is astoundingly broad. To the point that very mundane people have web cam covers for their laptops or the little piece of post-it note taped over. Like people, no one gives enough of a shit about you that they will hack your camera and even if they did what are you doing that you care so much? If the answer is nothing then you are the last person that needs to worry about someone hacking a fucking webcam.