Restoring Vital Injections?

[StandardFiend]

So, I noticed that occasionally after killing an enemy one of my Vital Injections would be restored. So far I haven't been able to pin down what exactly is triggering it. Does anyone know?

[Burusagi]

When you reach certain Tech Scrap multipliers, AFAIK.

[Phoenix]

It's when you earn enough scrap to level up. This gives you a few free refills on injectibles, or if you have none or are already full, a free filled energy bar. There's actually lots of little secret mechanics like this in the game, like passive health regeneration while near your scrap pile you dropped when you died.