I have a 98 Audi a4 Quattro the engine coolant never leaked all the sudden started leaking about 1/2 out of the reservoir every few miles I would fill it up and then now when turned off all leaks out?

Have you checked for damage to the upper or lower radiator hose? You can get a split that will leak when the coolant is hot and then when the engine is cool that split will close up enough where it won't leak.

Check the plastic manifold on the back of the head it's where the heater hoses conbect It's most likely it It keeps getting worse till its replaced Its not a cheap piece of plastic either