If you make an area with lava and flowing lava, then remove all the lava with /fill, there is some light that stays around a certain area.

How to reproduce:

• Place lava in an area with holes between the lava sources

• Let the lava flow in these holes
• Remove all of the lava with this command : "/fill ~-10 ~-10 ~-10 ~10 ~10 ~10 air replace lava"
• Remove the fire that could have been created
• Watch the light at the edge of the area