Coolant Leak
 

I have a slow coolant leak 2002 A4. Have to add coolant every 2 weeks. Taking my car into shop on Friday but wondering if someone knows what this is... I noticed the other day when adding coolant the coolant was leaking out from under the coolant reservoir as I was pouring it, is there a drain plug underneath the coolant reservoir? I felt where it was coming out and there is a hole that seems like it is supposed to be there. Thanks,