Be on the case
Definition 1 of 2
Pro Tip 1/3Colloquial Use
Used in everyday conversation, not in formal settings. 
The sink is leaking again? I'm on the case!
Pro Tip 2/3Taking Action
Implies the speaker is not just aware but actively working to solve the issue. 
You forgot the meeting notes? Don't worry, I'm on the case.
Pro Tip 3/3Reassurance
Often used to reassure someone that their concern is being addressed. 
Missing shipment? Relax, I'm on the case and will find it.