The valve on the CO2 bottle is what's leaking, and it's supposed to.
When you close the tank valve, you are unseating the top of it. A CO2 tank valve needs to be all the way open when in use, because it seats at the top to prevent leaks. When you close it, you unseat the seal at the top of the valve, allowing the pressure in the regs to slowly leak out.
This doesn't necessarily mean that your regs/tank isn't leaking, but it's not a conclusive test. I'd say if you have no bubbles, you have no leak.
Lemme know if this doesn't make sense.
Double edit: this applies to both posts above.
Edit: Damn OCD, I couldn't leave it with my half assed explanation....here's a picture: