Is it safe to stick an Apple Watch charger to a piece of metal when not in use? Maybe this is a general question for magnetic / wireless charging. I'd like to let my apple watch charger stick to a metal lamp on my nightstand when not in use. Is this safe or will the charger be activated in some capacity? Could be a dumb question, but I searched for this and couldn't find anything.
The short answer is you are safe as long as the ambient temperature and charge hardware don't overheat.

The empirical evidence is you can measure the temperature of the wall adapter and the charging puck to approximate the amount of current that's flowing through the inductive charger at any point in time.

I like to use a fancy infrared thermometer for things like this, but you can use your fingers to know that the charger is within specifications by comparing the temperature change when you take a charging watch off the charger and then let the charger sit against your chosen piece of metal.

Apple's official safety information are at:


https://help.apple.com/watch/ or http://manuals.info.apple.com/en_US/apple_watch_user_guide.pdf
https://manuals.info.apple.com/MANUALS/1000/MA1712/en_US/Apple_Watch_Info.pdf