In conductive grid pad charging applications, how does the charging base distinguish between human and metal contact to avoid electrocution?
This is covered in NEC 725.21, as a power-limited low-voltage circuit.  


voltage is 30V or less
total power is 100VA or less (therefore under 100W) 
both of these things can be true at different times, i.e. A charger could output 30V capped at 3A then later 16V capped at 6A.  


These are given relaxed treatment in NEC because of the low probability of starting a fire and the nil probability of electrocuting someone. 

It takes more than 30V to push electricity through human skin, unless you are doing something really weird like planting your face on the power base right after getting a bunch of fresh piercings.  

What's more, it isn't terribly hard to put arc-fault AFCI detection in a DC circuit.  If someone drops a set of keys on the power block, that will make a distinctive electrical "sound" (you could literally hook up a set of speakers and hear the sound).  So the power block can listen for energy usage that is not like a device being charged on battery.  Here enters UL and the other listing agencies, who have the power to make that a requirement for a listing.  You can bet a key/coin drop will be part of their testing.  No UL listing, no sales in the civilized world except through eBay and AliExpress. 

Hypothetically it would also be possible for the charging base to have a conversation with the device - especially in the flicharge.com example.  If three devices report drawing 70ma, 200ma and 130ma respectively, except the base is seeing 900ma outflow -- uh-oh!