Any idea why flex duct would be routed from beside an attic heat pump to a roof vent? 1st time seeing this.
Have a photo? Year built? Year of heat pump? Model #?
I attached a couple of photos. Home was built in 2013.
Did it connect to return plenum? Fresh air intake for tight construction.
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Is it ran from the HVAC return ? It may be a make up for a kitchen exhaust.
That does not look like it is connected to anything.
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You’re right as I zoom in it appears to be just laying on top of the insulation.
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I has never connected to anything, just laying there.
There’s no kitchen exhaust.
Either something changed… or perhaps it meant to serve a bathroom exhaust. We were not there 
if it was not connected, it’s anyone’s guess.
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Well, it might not be for the heat pump, or awaiting an air exchanger. Right now as it sits, it’s fairly static.
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Is the heat pump new, or original to the house, 2013? What does the vent look like on the roof ( exhaust or intake)?
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It’s original to the house. It terminates at a standard attic roof vent
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I would chase down all the bathroom vents. Once those are eliminated you are left with a maybe.
Fresh air can be added to the HVAC system, but the photo is lacking those components such as a damper and filter.
Here is a little something on the subject. I’m sure there are better links FRESH AIR FOR VENTILATION AND BUILDING PRESSURIZATION | Contracting Business
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I think Simon is correct.
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Likely it was fro a make up air vent that was never connected though it’s unusual to see it terminate through the roof. Maybe they forgot. I’ve seen them forget several times…Sometimes they forget a proper termination outside or to connect it to the system at the air return…It’s insulated and typically around here bath vents, dryer vents and kitchen exhaust fans are not insulated.