When you lifesteal, you get some green particle effects on you. This effect currently relies on the enemy to see you. So if you are not visible to the enemy, the lifesteal particles don't appear.

This makes no sense. Why should an enemy require to see you for the lifesteal particle effects to appear on you? It's misleading to the lifestealing player, making it look like Lifesteal didn't trigger.

This is mainly noticable on heroes with shared lifesteal. If e.g. your Spirit Bear attacks, you get healed from it, but you don't see the lifesteal particles. It's also noticable on heroes who can attack while invisible (Slark in Shadow Dance, or Riki with the level 25 talent).