Can you get a photo of the thermostat housing and surrounding area? It is very common on our cars that somewhere around 100k miles the inner wall of the plastic flange's coolant passage (between the t-stat housing and the head) will deteriorate and allow the formed sealing ring to bulge out--allowing coolant to leak.
It looks like this when it fails:
When mine failed I disassembled the aluminum crossover and manifold; scraped it all as clean as a dinner plate, removing all deteriorated plastic and other crud; degreased it all with denatured alcohol, and reassembled it using JB-Weld with no gaskets--that was 35k miles ago:
(I also bought a new FRPP manifold which sits in my attic, as insurance that the JB-Weld repair will never fail.)
Get a photo of it if you can, if it has failed as described above it will be obvious...
- 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum, Delta Force tuned,
Steeda UDPs, Ralco flywheel, RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.
New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq