Any guesses on what could make these stains at the roof vents? My first thought are bats, but I figured there would be droppings if they were using it as exit/entry ways.
What kind of insulation?
Air enters the attic then cools and then exhausts out the vent onto the hot metal surface causing a slight bit of condensation. This will attract some particulates, soot or even a bio growth. That’s my two cents.
Edit: That home looks old. Old attics are dirty.
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The roofer may not have cut a continuous ridge vent slot allowing a concentration of moisture in the two areas near the vent? Or the bath, kitchen, or laundry vent terminate in the two areas in the attic near the vent? 
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Fiberglass batts.
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Makes sense, it is a dirty attic, dirty inspection too, lots of vics under the nose.
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looks like very little venting for that much roof…
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I agree with Brian about this being an indicator of where the ridge vent is open. But I think the stains occur in cooler months.
Night and day, warm moist air escapes the ridge vent where sheathing and underlayment have been removed to allow for air flow. At night, When the warm moist dirty attic air exhaust hits the cooler metal exterior, condensation accumulates. During the day the sunshine and warmer weather dry off the roof but all the contaminates stay on the roof and make a stain.
From above you can clearly see which areas of the ridge vent are blocked by sheathing and/or underlayment, and which areas are venting.
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Moist air leaving the attic and condensing on cold metal and mold/mildew growing. Could also be some dust/debris from exhaust fans or just in the air as it’s leaving. Almost like a filter. In my area we have more standard square vents than ridge and it’s amazing how clogged with debris those things become after just a few years.
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Agreed, night time.
How did the attic space look. Was the insulation or sheathing dirty? Did a system vent into the attic? Any flue or ductwork damage in the attic?
The black dirt on the metal roofing, just outside the attic ridge vents, could be easily caused by a atmospheric particulates that can drift with the air.
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Pretty clean attic, bat droppings on one end. One gable vent, ridge vent and soffits, that were partially blocked by insulation. Bathroom and stack vents terminated to the exterior. No flue or duct work.
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I think it is basically ghosting/thermal tracking. But on the roof instead of the interior where it is typically more visible. The red metal roof covering makes it stand out more than typical asphalt shingles would.
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