There are several issues. You are asking two questions. First is a field gas chromatograph/mass spec device to analyse air samples and determine what compounds are present. Easy enough, although these machines are not necessarily very portable or very fast, the tech certainly exists, especially if there are only a few things it looks for (NBC detectors for the military are an example if small, portable detectors that just look for a few things).
But being able to visually sweep a room and "see" odor is another problem. Odor is really just very small organic and inorganic compounds and molecules that have to have physical contact with olfactory receptors in order to generate a "smell". The range of compounds is far, far greater than the visual light spectrum humans can distinguish. So you can't map all the olfactory range on-to the visual spectrum. Put perhaps you could map select compounds on the fly (like the Batman sensor).
But how to detect the compounds at distance? We smell at a fixed point and use some basic analysis to determine a direction, mainly detecting increasing or decreasing concentration and cross-referencing based on air flow and what we can see that might produce that smell. You can't "see" a source of odor, you can only detect it at the point it contacts the nose and follow it based on intensity. So while Batman can "see" footprints or whatever as if the odor source glowed, in reality you would have a gieger counter-like variable intensity light haze that increased in brightness as you approached an odor source. A Geiger counter is detecting particle concentrations and mapping them to an audio spectrum (tick frequency shortens as concentration rises), "visual smell" would be a haze in your vision that only extends an inch or two in front of your eyes but grows in brightness as you near the targeted odor compound.
Your only other option would be to make the target compound fluoresce in some manner. You would need to be able to emit radiation that only interacted with the target molecule and caused it to emit EM radiation itself (doesn't have to be in the visual range, just some wavelength your equipment could detect and map to your visual spectrum). Kind of like a black light, the target molecule "glows" only when the search beam hits it and excites it. This would let you see things from far away. I don't think many odor compounds could actually do this, but you could handwave it. It's been a while since I've played a Batman Arkham game, but I think this was sort of how it worked.
To speak to some of the comments, the way I would envision visual mapping of an odor would be a peripheral haze or rising bar on the edge of vision. You select the odor compound to be tracked and set a "zero" based on the concentration you are detecting at the moment. This starts at, say, 25% intensity but could be based on a known lower limit of detectability for that specific compound (most analysers have an upper limit as well which would require diluting the compound in order to lower it back into range, but we can ignore this for now. If it drops in concentration the intensity reduces from 25% to wards 0%. Of course in the real world there is no way to know that 100% equals "right in front of the source of smell" like in a game, but presumably you could set the scale based on numerous real world examples so that 100% would encompass realistic levels of smell exposure or the scale could be logarithmic in order to handle an unknown upper range of compound concentration.
AS for emitting a EM beam that detects compounds or causes them to fluoresce, I can't actually think of many examples other than the known excitable compounds we already have. If I had to handwave a technobabble solution (which I have no idea has any basis in reality) I would have a brief intense IR heat pulse that creates a pseudo flame photometric effect that could be detected at range. Sort of an "odor radar" if you will. With a high enough sensitivity and resolution, you wouldn't need to ignite much in order to detect the compounds, so you wouldn't have to set everything on fire just to measure the spectra.