Your phone's (rootkit?) would have to passively measure the room before any remote action, that's true and reduces the utility. Still it would require accessing the sensors in the faraday cage which might make some noise.

Plenty of VM evasion stuff has been caught in the wild so it would up the game regardless, which is all you can really hope for against hackers and malicious parties. They almost always go for the easy targets who don't think of this stuff anyway.

Good point none-the-less.