I recently purchased a 2009 S coupe and after only driving a few hundred miles, I noticed the upper radiator hose was sucked flat after sitting overnight. The previous owner had replaced the t-stat & lower hose, so I had my mechanic do a radiator flush, new oem cap and upper hose. Now the problem seems to be worse, after the engine cooled down I checked the hoses and both upper and lower were completely collapsed. When I opened the cap there was no coolant at all in the rad or the hoses, it was all still trapped in the overflow tank (about an inch above MAX line). Somehow coolant is not being drawn back into the rad after cool down. Temp gauge reads normal and heat works normally. Has anyone experienced something like this before? Thanks

 

Usually this is due to a CAP however you state OEM cap was used.
It's quite possible that if the cap was good then there was a huge air pocket in the engine.
If this was not the case then the cap is bad
If you cannot confirm an OEM thermostat was used then dont remove it from the list of suspects just yet.