The first comment in the thread you link to says the Raspberry Pi connects to botnets and records all network traffic.
One wonders how it does that.
Sort of tongue in cheek, since I don't know the range of state of the art acoustic side-channel taps. I guess you'd also probably have power fluctuations and network timing channels to exploit.
Plus, a lot of different points at which to attempt to insert a second stage into the connected devices themselves, using all the tricks everyone else in this thread has mentioned.