> You should realize that aluminum foil blocks visible light signals.

Yup, I mentioned the solid box earlier - a non-mesh metal container.

Reading through some more material, it seems that there are ways around it - one can use a static electric field to send signals, e.g. switching it on/off or changing its intensity. I suppose the differing el. charge of the opposing walls of such a cage/box would create en electrical field gradient within such box, which is easy to detect, if one's goal is sending some low-bitrate signals.

It's detectable (i.e. the fact of using it), but the communication scheme itself can be encrypted or encoded, so plausible deniability can be employed.