This 45° elbow is the one that's leaking from the upstairs sink. - It's clear from this shot:
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[*] There did used to be a bathroom of some sort upstairs
[*] The inside of this wall got rather wet at some point in the past -- Update: This is due to the leak in the upstairs sink drain.
[*] Wall construction is smallish sheets of drywall with thick-ish plaster over top
[*] Some plumber got his torch a little close to the wall
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