I've made a series of penetration tests in my network and one of the things I've tried was to record webcam and microphone.

Recording an end-user's microphone seems to be a stealth thing, but what about the webcam?
In my tests, the indicator is turned on and I can't figure out a way to do this without turning on the light.

So far, I'm assuming that if someone broke into my computer and turned on the webcam, I'll know that.

But, if that's possible, which of the available hardwares on the market are vulnerable to that kind of attack?

Most definitely,  but in order to do this you would probably have to patch the camera's firmware and then flash it.   Similar attacks have been used to disable the "shutter sound" on cameras. 