This is a general question. Nothing to do with Arduino specifically. On many occasions, I just leave my smartphone and laptops charging even though they are fully charged. Will the charger still draw current and waste power after battery is fully charged? Is there a danger of over-charging and risk of explosion?
Yes they draw current, and probably no (increased) risk of explosion (chargers for lithium batteries should
know about safe limits of lithium batteries).
Most chargers go into a trickle-charge mode after full-charge. With lithium batteries any trickle charge
has to be very very low current, so the charger probably stops sending current to the battery, but
of course it is still drawing current itself (its a whole circuit plus microcontroller, with a mains SMPS that's
permanently on.
A good SMPS might only draw a fraction of a percent of full-load current when there's no-load, but its
still some current. But different chargers are different, you need a power meter to really find out if
the specifications don't mention this.
The temperature of the charger is a good clue to how much power is going through it, they are around 80 to 90%
efficient under load so will get noticable warm - but should be a lot cooler when not charging.