It looks like it may be an automatic air vent from a plumbing fixture. Plumbing fixtures are usually vented through the roof with an open pipe, but if it is difficult to extend an open pipe through the roof, it is acceptable to use an automatic air vent device. Many kitchens have these located in the sink cabinet below the countertop to vent the sink drain, but they can also be used in the attic, when venting through the roof is not convenient. Although, if it is an automatic air vent, it would have been better for the builder to connect the vent to another vent pipe that extends through the roof.
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