Continuing in a series of strange things that I have seen while consulting at hospitals…
According to NFPA 72-2012, Annex A.220.127.116.11 smoke detectors should not be located in direct airflow, or any closer than 36-inches from an air diffuser. This would include return-air diffusers, exhaust-air diffusers, as well as supply-air diffusers.
While the Annex section is not part of the enforceable code, it is explanatory information provided to give the reader direction on how the Technical Committee viewed certain standards. Any authority having jurisdiction (AHJ) may use this Annex information in determining compliance with the standard, and most AHJs do. Therefore, the 36-inch rule is widely enforced in all surveys.
But some smoke detectors have UL listings for use in high-velocity airflow areas and are recommended by the manufacturer to be mounted close to air-diffusers. This would lead one to believe they would be permitted within close proximity to an air-diffuser, such as the one in the picture indicates. But I have read reports where surveyors still cite the organization for installing the special high-velocity smoke detectors too close to air-diffusers because of the perception that the airflow would prevent ambient smoke from being detected by the smoke detector.
Rather than fighting this battle with over-zealous surveyors, it is best to just make sure all smoke (and heat) detectors are at least 36-inches from all air-diffusers.