Exhaust Gases in Coolant?

I recently experienced some coolant loss with my 1994 Camaro Z-28. After much fooling around, a local shop advises that my heater core is leaking. They also said they did a sniffer test and found engine gases in the coolant. They suggested this could require a $2,000 repair, involving exchange of the heads for rebuilt models.

This all sounds rather fishy to me. The amount of coolant loss is rather small and could be easily explainable by the defective heater core. There has been no change at all in the car’s performance. There is no sign visible sign of corruption on the dipstick. Is the mechanic trying to pull a fast one on me? I told him to replace the heater core, but ignore the other problem. (I liken the situation to a doctor telling me I need to have a heart bypass operation because my cholesterol is too high.)

Should I be concerned? What should I do? By the way, this car has 52,000 miles on it and it has never been abused.

If they sniffed the coolant for hydrocarbons, and it was found, that pretty much points to a head gasket problem.