What household appliances are most likely to set off a Geiger counter?

Apparently microwaves, electric blankets, humidifiers, vacuum cleaners, and hair dryers. But my source is a bit suspect.

Electric blankets are showing up on a lot of lists.

I tried the microwave, no dice. Or maybe faulty Geiger counter?

Granite countertops
The mantle on camp lanterns
Concrete itself, sometimes (even though it’s also a shielding material)
Coal piles (if you have any of those nearby)

The microwave is not producing “ionizing” radiation.

@WasCy You don’t need to put ionizing in quotes. You’re quite correct in that microwaves are not radioactive.

There is one common household appliance where you might pick something up—your smoke detector. It’s incredibly small, but it’s there. http://www.epa.gov/rpdweb00/sources/smoke_alarm.html

Try it out, I’m curious if it detects it.

Man, you are so lucky.

The glowing numerals on some watches.

Most older smoke detectors contain Americium-241 . Open up the detector and look for the metal shield about 40mm in diameter. You will find what you seek under that cover.

Check the bottom of the detector to make sure you have the right type. Use a magnifying glass as the label is written in 4 point type.

This may be slightly impractical, but if you REALLY wanted to check it’s accuracy you could go to the airport and hop a plane to Japan.