Another 'sort of' answer:
One possibility, more analogous to a traditional camera, is to use a stationary receiver and a strongly directional antenna. If the antenna is directed in the same way that an electron beam moves across a CRT screen, a render of signal strength can be created that can then be overlaid with a photo taken from the same point. While the parts are readily available (see wikipedia/cantenna), I haven't come across a project or commercial solution that uses the cantenna as a camera in the way described above.
As @Michael noted, this probably wouldn't give you a 'good' image: radiation at these wavelengths behaves differently to visible and near-visible light. Rather than simply behaving differently depending on the relevant surfaces, radiation at these wavelengths is more measurable as amplitudes per point in a 3d space. The question uses a key word: the room or space is truly flooded.
Youtuber CNLohr provided an explanatory video showing how to measure transmitter power from a single WiFi source using relatively low-cost components.
This isn't a "camera" as such, even though a camera is used to translate the signal from point measurements to a 3d image, one vertical layer at a time. However, it does give a (3d) image that can be flattened and overlaid onto a normal photograph. On the downside, it relies on moving the sensor through every point in the space to be imaged; not exactly a 'snapshot' measurement.
It's conceivable that this design could be adapted: the sensor could store position information based on an indoor GPS and record its own data, rather than needing a camera. The software can also be adapted to measure the total signal per point instead of simply the signal from a single transmitter. When selecting a wireless signal, a list of identifiable signals and strengths is presented.
I believe this would give an aesthetically better image than directional measurement; however, like the directional antenna camera, it's not available as a commercial product.