For the detector placement, is there a list here already of items and locations within one's dwelling / building to avoid?

Clearly the idea here is to provide a measurement which is not already being influenced from something radioactive within one's dwelling, such as Krypton-85 in building windows, granite counter tops, etc.

Florescent lights and smoke detectors (from the roof on the floor below if you place it on your floor) may contain radioactive matter and can increase count rates. Basement will get a lower count than top floor.

Florescent lights: Are they react with the detector too, when they are off?

Whats the minimum distances to these things like the light and the smokedetectors?
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DSKAG Austria Research Team: http://www.research.dskag.at

the smoke detectors contain very small amount of Americium-241 so its activity is around 37 000 Bq, according to this source:
http://www.world-nuclear.org/info/Non-Power-Nuclear-Applications/Radioisotopes/Smoke-Detectors-and-Americium/#.Udv7cpzYeqs

only 0.3 micrograms. They also say:

Am-241 emits low energy gamma rays of 60 keV. The Am-241 gamma dose constant of 3.14 mSv/hr at a distance of one metre from a certain amount - 37 GBq - of Am-241. This gives an annual dose at one metre of 27 µSv/yr for an average household smoke detector – around 100 times lower than the dose from natural background radiation.

For smoke detectors: Not all have a radioactive source, mine don't have it. It's just in cheap, old stuff, good new stuff is optical. They are problematic, as they have to be disposed as radioactive waste, and must not enter the ordinary waste bin, at least here in Europe.

Edith says:
Old TV-sets and monitors, those with CRT, not LED/LCD, also emit radiation. I tested it with my sensor once, and it quadrupled the amount shown in the display if I got the Geiger-tube within a few centimetres from the screen.
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Gruesse vom Saenger

Thanks for the replies!