It sounds like you had your X in for something else and indy mentioned thermostat or did you have symptoms that tied to cooling system?

Since you are only driving short distances it should not harm the engine and it is possible it does not get to proper operating temperature with good or bad thermostat anyway. Regardless, it is not something that has to be done right away.

I would think if the indy wanted to scam you they would have chosen to repair that costs a lot more and requires many hours of labor. If you are concerned about integrity of shop either unlock the OBC and monitor temperature or ask the shop to do so. Then let the engine idle for 15 minutes to monitor temperature before making a decision to replace the thermostat. If indy is trying to put one over on you it will do no good to ask him how he knows it is a bad thermostat. I would either monitor the coolant temp or get a second opinion and prices from to do the repair.

Code or no code, light or no light you will have your answer.

__________________
X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas