It has a power I/O rocker switch. When flipped in the I position, it lights up, plays ascending tones, then connects to the last device. When flipped to O, its lights go out, it plays descending tones, disconnects, then stops everything. Is O truly off?

 

It is very likely the switch does not interrupt the power supply to the circuit and instead is connected to an input on an MCU signaling it to disable the device.

But, there is no way to know that without seeing where the switch is connected.

 

Define "truly."

One gizmo I made needed to be very low power except when as it was working as a battery powered pump timer. I don't remember what exact part I used but there was a LDO regulator to convert the 12V to the 5V a micro controller needed. This also worked as the power switch. The single multipurpose button on it when initially pressed would engage the shutdown pin of the LDO, which powered the micro, which in turn would then also drive the LDO shutdown pin to keep the LDO on without the user's finger until later. The very first LDO I just Googled up, the TPS709-Q1 by TI states:

These regulators can be put into shutdown mode by
pulling the EN pin low. The shutdown current in this
mode goes down to 150 nA, typical.

So is 150 nA (0.00000015 amps) close enough to "truly"?

 

It has a power I/O rocker switch. When flipped in the I position, it lights up, plays ascending tones, then connects to the last device. When flipped to O, its lights go out, it plays descending tones, disconnects, then stops everything. Is O truly off?

This behaviour (tones, etc.) happening after the rocker switch is flipped to "O" tells me the switch is indeed connected to a microcontroller pin that informs it the user wants to turn it off (as @Yaakov said).

To indeed see if it is a power off, perhaps you could do a current measurement on its batteries.

 

This behaviour (tones, etc.) happening after the rocker switch is flipped to "O" tells me the switch is indeed connected to a microcontroller pin that informs it the user wants to turn it off (as @Yaakov said).

To indeed see if it is a power off, perhaps you could do a current measurement on its batteries.

The shutdown sequence only happens in the wireless mode. In the wired mode it instantly stops everything without tones when the switch is in the O position.

 

The shutdown sequence only happens in the wireless mode. In the wired mode it instantly stops everything without tones when the switch is in the O position.

Ok. So one more reason to believe this is microcontroller-based control. Unless the switch between wired and wireless is also done via a mechanical switch...

 

To determine if it is truly off when "Off" instead of switching it off and getting the descending tones, just pull the power plug. If it goes off immediately without a tone then the answer is "No, it's not fully off when 'Off'.".

My BT headphones - I have to hold the power button five seconds. Then I get an ascending tone and it comes on. When I hold the button an additional five seconds I get the descending tones. If I leave it unused for months - when I come back, it's battery is dead. Needs to be recharged.