Coolant leak?

When waiting at a traffic light I notice my temperature starts to creep up. But once I either start to drive or rev the engine a little and returns to normal operating temperature. On inspection I have noticed a small puddle of coolant on top of the transmission, which sits right under the thermostat housing and hoses. The check engine has not come on, but I have noticed the air conditioner seams to turn off when it gets warm. Put if I turn it off and on it comes back on again. Thinking it's leaking from the thermostat housing, also there is a slight smell of coolant outside the vehicle but not inside.