Strange leak from exhaust

Hi all, here in the UK the weather has gone cold. At the same time I've just had my Camaro serviced. I've noticed recently a lot of black fluid leaking from the exhaust, just as it attached to the back box. It's doesn't seem oily, but plenty of it especially when I first start it. Is this condensation or am I missing something?

The only thing I've noticed, is the bottom section of my engine coolant was always empty, they topped it up to the half way line on service, and it has decreased since, which I'm worried could mean head gasket...

Any thoughts?