Day One-thirteen: A clue, but to what?

So if we haven't mentioned it before, the new house has had a confounding odor. We figured that it was the 25 year-old wood-burning and creosote-belching stove. That's long gone and the smell remains. Then we thought it must be the ceiling tiles in the basement that soaked up the creosote. We'd replace those with new drywall and be all set. Nope. 


The smell was fairly localized to the side door entryway and the stairway to the basement, so we thought that once we pulled up the tar-covered (for waterproofing) landing in the entry and installed new, maple stair treads that we'd resolve this for good. We did all that and still the smell persisted. What the heck was going on here?


One other place where the odor was strongest was in the little basement side room where we discovered the dampness and mold. A-ha! This must be the root cause. So we've removed all the moldy wood, bleached the heck out of everything in the area, and taken several steps to waterproof the room (cement caulk along the cinder block-to-slab seam, hydraulic cement into identified gaps). Clearly, success awaited us. But still there was something lingering. 


So tonight, while poking around in that basement room, we noticed something we hadn't before and couldn't when there was panelling up--one 2x4 with an odd discoloration running the whole length. We're pretty sure it's not mold because there's no water source and it absolutely covers the surface of the 2x4, so we're a bit at a loss. But one thing's for sure, we're fairly certain this time that it's the source of the odor. Any ideas from the crowd?