Much air cargo gets loaded on “unit load devices”, e.g. (metallic) aircraft pallets and also in cargo containers like lower deck containers and igloos. However, a lot of said cargo is trucked into the airport on smaller wooden pallets, and that cargo is often forklifted (together with the underlying pallets) directly onto 125 x 96 inches aircraft pallets and the like. So a lot of wooden truck-type pallets fly without this being noticeable to even attentive passengers, I think. The wooden pallets serve to keep the load forkliftable, not only for assembling the aircraft pallet load, but also for subsequent surface transport.
Even though a wooden pallet could be on an aircraft, it doesn't mean anything, because wooden pallets are all over the place, all over the world. I'd be surprised if they didn't find one!