Turns out one exists [2] (in theory—no commercial product a decade later), but the one I was thinking of would use microphones rather than cameras [3] to detect the bloody suckers. Thing is, I can't find any prior research or even hints that people have considered this approach. (The closest I can find is an app [4] that detects a mosquito from ≤10cm away.) Here are some of my guesses for why it's not been done:
A. Maybe it's too difficult to distinguish mosquito buzz/whine from background noise, even when filtering in only known mosquito frequencies (~400Hz [5]).
B. Maybe it's too slow to sweep a room with a directional mic that's focused enough to detect mosquito sounds? (Say the turret in one ceiling corner of a 12x12x8ft room.)
C. Maybe directional mics don't work well for all sound in their cone and so need to be able to "focus" at different distances (by dynamically changing the parabolic dish's shape), thereby increasing the room sweep time?
D. Maybe the parabolic dish needs to be so big that it just looks ridiculous?
E. Maybe all of this is possible but the type of microphone is so expensive that it's worth pursuing commercially?
See also this research paper: Detecting Insect Flight Sounds in the Field: Implications for Acoustical Counting of Mosquitoes [7] (I'm pretty sure this is not directional—they hung an attractant 15cm from the mic and I don't see any dish.)
[1]: https://www.youtube.com/watch?v=BKm8FolQ7jw
[2]: https://www.ted.com/talks/nathan_myhrvold_could_this_laser_z...
[3]: I don't think cameras are well-suited to detecting something so small. The "photonic fence" from the TED talk only kinda works by adding a white background behind the bugs so it's really easy for the camera to detect them—not that practical in a normal room.
[4]: https://www.theguardian.com/science/2018/mar/29/mosquito-ear...
[5]: https://www.youtube.com/watch?v=Rcm3rPz_Q94&t=127
[6]: https://mysteriousuniverse.org/2014/08/why-mosquitos-are-hum...
[7]: https://pdfs.semanticscholar.org/a0bf/4fcd478b6214759046a739...