How do you earn "banked" Mana?

After a couple of matches, I've noticed I had 78 mana in my bank. I haven't sold any items or banked any using the forge. Are there other ways to earn Mana?

Here's from the wiki:

• It is possible to bank mana during the building phase using the forge. (Only after first wave.)
• Items left on the ground at the start of a wave get turned into mana and split between the players.
• End of Level - At the successful completion of a level, you will receive some mana back for your towers.
• Upon leaving a level - Items on the ground are sold and split between the players
• Extra items can be sold for mana based on their quality.
• Mana can be traded between heroes in the tavern by dropping it.
  • Access the Hero menu and choose "Drop Mana".
  • Mana has a maximum of 100,000 per Drop.