What Is The Risk Of Keeping An Extension Cord Or Adapter Plugged In?
In general there is minimal risk from leaving your charger plugged in. Because the charger only draws current when it is in use. It's always possible that the charger could malfuction and expell current even when a device is not connected, this is rare but there are some documented cases of non-UL (UnderWriters laboratory) tested and certified devices that have exploded.
See this article: https://gizmodo.com/dont-buy-shady-cheap-chargers-unless-you-want-your-ipho-1788064029
However, the chargers do draw power (notice I didn't say current) when their not plugged in because the transformer is dissipating a small amout of energy it's receiving from the outlet. That alone should be a compelling case to unplug your chargers. For instance if you owned 10 adapters (phone charger, laptop charger, etc) that have transformers in them and they consume 5 watts each, that means you are wasting 50 watts continously. If your cost of electricity is 12 cents kwh per hour (0.05 kW*24*0.12) then you're spending approximately $52.596 more in electricity each year. Thus it's best to keep your adapter unplugged!