So.. googling for the model number (WNW777DC-A), I found a link which suggests it is a rebranded "Family Guard FG888D". The user manual for that device, says it works using "ionization technolgy." So I imagine the test circuit generates some of the appropriate "ions" for it to detect.

On a related note: anybody know how to de-Googlify a Google search result? The link above is shrouded in googlisms, and I really just wanted to post the true link to the maker's manual. How to do that?

Edit: found a Firefox plug-in for it.

Thanks.