Unmodified and Pd modified Zinc Oxide (ZnO) hexagonal nanorods, grown by Chemical Bath Deposition (CBD), is reported in this paper for efficient detection of acetone vapor. After details structural characterization (XRD, FESEM and AFM) the nanorod based sensors were tested in resistive mode for detection of acetone in the concentration range of - 190-3040 ppm. By Pd surface modification the optimum working temperature was brought down from 350 degrees C (unmodified) to 300 degrees C with appreciable improvement in response magnitude (90% to 99%) also. Strikingly, the recovery time, after Pd modification, became faster than the corresponding response time up to certain concentrations range (190-1530 ppm) and above this concentration (> or = 1530-3040 ppm) response time was found to be faster than recovery time which is similar to the case with unmodified nanorods (for entire concentration range). There are earlier reports on such faster recovery (compared to response), but no proper explanation was provided. In this paper we tried to explain this apparent anomaly of recovery characteristics through concentration dependent reaction rate variation following Arrhenius equations. Also a correlation between the parameters of the corresponding electrical equivalent circuit of the sensor has been established.