If you are going to have a leak it will be the smaller hoses that run from the idle air control valve to the throttle body and across to the intake manifold, check the ends of those hoses for any greenish stains. They can leak small droplets that evaporate quickly and often don't make it to the ground.

 

Can I change it without bleeding the coolant system?

 

would you happen to have a link to the overflow tank you bought?