Coolant leak when cold

I have a slow coolant leak that seems more noticeable when car has been standing. Sent it in to my garage suggesting it may be water pump as was dripping from bottom of pulley front centre. Picked it up and was told top hose was perished so replaced that!!! That evening spotted leak again so had a moan, took photos of drip and at further expense they changed water pump yesterday and checked for leak this morning after been standing. Picked it up today and have driven about 10 miles. Just looked (7:15 pm) and still dripping so where can it be coming from?
Rad and pump originally changed about 19 months ago.

2.5 DHSE

thanks in advance.

Thermostat is a possibility, why doesn't the garage carry out a static pressure test and then look for the coolant leak? It should be at the garage expense as they haven't fixed the leak that they missed diagnosed!!!!

Possibly the metal pipe with an O'ring on that goes just below the thermostat housing.