There have been some reports of government-sanctioned malware being installed on smartphones to override the behavior of the power-off switch, making it appear that the phone has been turned off when it's actually not. This could presumably be used for surveillance purposes, and probably has been, given what we've seen in the Snowden disclosures and elsewhere.
However, the idea that an unmodified or unhacked phone can be made to behave this way is sheer tinfoil hattery. You're not allowed to bring anything even vaguely electronic into a SCIF, but that's because the whole idea behind a SCIF is to have a room in an otherwise-unsecure facility where even the vaguest potential security risks are aggressively countered.
Baseband alone is not enough to act as an eavesdropping device. There's no direct connection between the baseband chipset and the microphone and camera. And if they left the baseband subsystem powered up when the phone was physically turned off (as opposed to in standby), the battery drain would be obvious.
Some of the confusion probably arises from the distinction between power-down and standby modes. If anything but the supervisory processor that monitors the power button were active with the phone turned all the way off, it would be noticed, and give rise to a substantial (and well justified) outcry. Under normal conditions, that can happen only if the phone has been hacked.
What I personally am talking about, would apply when the phone is powered on, for example while it is in my pocket, available to receive calls or texts.
However, the idea that an unmodified or unhacked phone can be made to behave this way is sheer tinfoil hattery. You're not allowed to bring anything even vaguely electronic into a SCIF, but that's because the whole idea behind a SCIF is to have a room in an otherwise-unsecure facility where even the vaguest potential security risks are aggressively countered.