Don't forget the possibility of data being repeated by your electric meter even if you don't provide WiFi. They're generally dual band, operating at WiFi frequencies for local interaction and with more power at a lower frequency for the networked traffic. While the protocols are not intended to be WiFi compatible, the already-proven-hackable devices are just a patch away from unexpected functionality. The local link is intended for things like telling your air conditioner not to run part of the time when the grid supply doesn't have much excess capacity.

What other data is sitting around? How about raster images of whatever your "copier" printer fax has handled? Even if you're using USB, if there's WiFi capability in there, assume it can be awakened. A link that's not compatible with your sniffer is a possibility too. Drag out a spectrum analyzer and directional antenna. While you're at it, check those new LED streetlights for bursts of r.f. too.