I don't own either. Do either broadcast a (bluetooth?) signal that they are activated and available? As opposed to being turned off.

My concern being, an unknown / unseen device could record your conversation - even if Alexa / Siri were off - and then covertly upload that later. Yes? No?

This is going to get ugly.

Being able to detect whether the ‘real’ device is listening won’t help much there, won’t it? The villains wouldn’t be so stupid to copy that feature to their recording device.

Also, you’re safe as long as you don’t buy and configure such a device (that may change if speaker recognition ever gets so good that devices other than your own will recognize your voice and bill whatever you ask them to do to your account)