Considering this is recording (or at lest intercepting and processing in some form) audio in a place where most people have an expectation of privacy, I assume you have to inform the renter that this is on and monitoring....
Guests' privacy is always protected. Our patent pending technology ensures that no content is recorded
It's patented, so it must be good.
Sarcasm aside, I really wonder what's in that patent... I mean it must (should) exist, otherwise it should be "pending".
[1] https://www.schneier.com/blog/archives/2008/06/eavesdropping...
A more practical side channel attack would probably be through radiated emissions over the mains power. However if you're willing to go that far, a laser microphone is much more effective and simpler.
Actually assuming it won't is something I'd consider naivety / lack of tech-savviness.
And while this device says it doesn't record, it doesn't say "it can't record" and it doesn't say "it cannot stream the data elsewhere".
Of course it would be easy to build something like this yourself, if you fancied a challenge. A mic-input, an ESP8266 chip, and you've got yourself an instructibles-project!
Sending an SPL every few seconds is a lot less bandwidth. It's engineering overkill to do anything else.
It doesn't help much if you are mostly spying on foreign tourist guests, since the act of visual spying of a private household is itself a serious crime - not only as a tort against a victim, but a criminal offence likely of interest to the public prosecutor.