Airport security checks and alike is the answer.
I recently flew to Berlin, so for the sake of science and personal learning I gave it a try. I obscured the smartphone camera as described in the App manual. Please note that you need two layers of black cardboard to be effective even in full sunlight. I fired up the App, slipped the smartphone in a pocket and waited for my turn. The jacket went into the usual box and through security scanning. I tried to stay calm when the operator at the screen asked for a second pass of the box containing my jacket. Got caught? No, I had simply piled up stuff and they couldn't see clearly everything. I grabbed my stuff with a nice smile since my counter disguised as a smartphone passed twice into the radioactive area!
This is the screenshot of both passes. The App log keeps track of hits per minute and probably the second pass was just across the minute change:
The two bars measure about 25k and 10k CPM.
Let us move forward. Upwards, actually, to the sky. When airborne we are exposed to higher doses of radioactivity because there is less air filter between us and the outer space. So, probably, the App could detect something during the flight too. Note 1: flying is perfectly safe with regards to radioactivity!! Note 2: the app ran for 21 minutes on each flight, but I kept the phone screen hidden under a paper or in my pocket.
First flight was before sunrise:
And the second flight after sunrise:
Not a big difference between the two airborne situations, but I can tell that I get fewer CPMs when on Earth.
Last but not least, I dared the security check experiment on the way back, thinking that it would have been fun to explain German security personnel what I was measuring and why. Well, I had plenty of time before boarding. I have no screenshot to show since during my stay in Berlin I needed my smartphone camera and I had no means to cover it back. So I simply fired up the app and threw it in the internal pocket of my Winter jacket: darkness in there is enough not to produce false hits.
The X-ray scan produced 17300 clicks, which is interesting.
Incidentally, the sum of both scans at my home airport is around 34k. Since the phone had gotten Xrayed twice, that makes an average of 17k CPM over those two minutes, which is very very similar to the third scan on the way back.
So. The Radioactivity Counter app does work. I do not claim it can produce calibrated uS/h readings, but it can detect ionizing radiation above normal/natural levels. Just let it self-calibrate, provide a good black screen (try it aiming the camera towards the Sun while the app is running, and look for 0 hits) and try it in a probably radiation free environment (your home, your basement) so that you know what is normal for your specific App/smartphone combination.
Good luck in not finding radioactive sources in your neighborhood!