Dave Plowman wrote:

wondered if any of the computer savvy types could explain a few
things I've observed with my voice activated Samsung TV.

[Tinfoil hat mode on]

1) If it is in standby, it no longer appears on my LAN.

It could be programmed to look invisible when "off".

2) I'm not aware of a method of bringing it out of standby over the
internet. (it can be set to look for software updates etc overnight,
though)

It could have a WakeOnLAN function, this would of course require your
router to also be hacked to enable directed subnet broadcasts, or the TV
could quietly poll a CIA server when it's "off" and ask if they'd like
it to turn on in snoop mode.

3) The LED on the front of the set changes colour between standby and on.
Think most would notice if it was on when meant to be off.

LEDs can lie, e.g. laptop cameras can be enabled without turning on the
camera LED, black tape over the lens works.

4) Can the really very cheap microphone fitted to such things
differentiate between the speech or music etc from the TV's own speakers
just a few inches from it and pick up intelligible speech from the other
side of the room?

once the audio arrives to the CIA's server farm they can throw all the
audio processing they've got at the muffled audio to clarify your bomb plot.