From what I'm reading, it appears to occur in about a third of all ICU admissions. Septic patients are statically more like to develop SIADH. Please see references below.
Regards,
Christopher
Pasha, S. A., Pasha, S. A., Prabodh, V. S., Vidya, S. D., & Suhasini, T. (2016). Frequency of Hyponatremia in Critically ill Patients. Indian Journal of Applied Research, 6(6).
Padhi, R., Panda, B. N., Jagati, S., & Patra, S. C. (2014). Hyponatremia in critically ill patients. Indian Journal of Critical Care Medicine : Peer-Reviewed, Official Publication of Indian Society of Critical Care Medicine, 18(2), 83–87. http://doi.org/10.4103/0972-5229.126077