What module to look for if I want to tell if there's Wi-Fi in the air?
Regular 802.11 Wifi networks use something called a beacon frame to transmit all of their "housekeeping" information. This includes info like the SSID, the current time, and the supported data rates. It gets transmitted every 102 ms.

WiFi receivers can only detect nearby networks by watching for these beacon frames. There is no difference between receiving ordinary data and receiving beacon frame data - it's transmitted over the air in the same way, so there can't be an SSID "sniffer" without a full-on receiver. That means that you really need a proper WiFi module with a regular receiver.

With all that said, the lowest RX current that Digikey sells in a WiFi module is roughly 2 mA. As Wouter said in the comments, you may be able to make your module hibernate to save power, but while receiving, you're pretty well guaranteed to be spending quite a bit of power searching for WiFi. I think your engineers need to go back to the drawing board.

PS: you said that the GPS module consumes 27 mW/h. This number doesn't make sense! Watts are already a measurement of energy per time. I think you mean 27 mW.