While I was traveling to the Hawaii and back by plane, I got three unusually high readings, the highest over 15K.
I was perplexed, and sought the advice of Vince Mazur, who makes the instrument. He was very helpful and spent a long time on the phone. He said the most likely cause was X-Rays. I reviewed the data, and found that my memory of the events were wrong. The photo of the Geiger Counter on the iPhone was able to show me the location and time of my data collection. The spikes corresponded to the Geiger Counter being run through the X-Ray scanner at the airport. The Geiger Counter had been turned off on the flights out.
Since 300cpmX60min=18,000 counts, then this exposure corresponds to about one hour of air travel. Also, since there is shielding around the X-Ray machine, the actual exposure to passengers walking near their baggage in line are exposed to much less.
If some one is collecting data about X-Ray exposure in airplanes, they should be aware that spuriously high readings will occur if the Geiger Counter is X-Rayed at the air port.