I have a Heil heat/air gas pack(forced air) unit. It's about 20 years old, but it's been a great one. When all the thermostats are off but the blower on the furnace has kiked on and won't go off in the attic since it has become extremely hot, everything works fine both heating and ac. What do you think? Could the attic heat trip a sensor on the unit?

This is a common problem where the fan contactor can stick. Sometimes it's an easy fix and other times it's harder. You either have to replace a relay, a furnace board or a fan center.