Ceiling stain, what is the source?

There are two lines that are about 2-3 feet long on my living room ceiling. The lines are rust colored. and about 4 feet apart. I had the roof inspected 4 months ago when I bought this house and it was in good condition. Above the rust colored lines are a decent amount of insulation and some vents. The AC and furnace are close by. It is a one story house, so the water heater, furnance, and A/C are in the attic. I spent about 5 minutes looking and could not find the source of the stain. Any ideas where else to look? Assuming I can't find the leak, what type of professional should I call for help?