Loosing coolant (but in a weird way)
 

Hello all
New to the forum, hope this is the right place to ask for advice
I have a 2008 Mazda 3. Since I got it (about 2 years ago) it is loosing coolant fluid constantly. The mechanics in the garage can't find a leak, but they replaces the container, the thermostat, and more. lately i noticed that the leak happens when I start my car. Not while driving or Idling, only while starting. Any Idea what could cause such a thing?
Thank you
Oded