TOPIC: Moisture staining on the common wall. Any one see this before?

Were they leaning a mop against the wall in that area?

I'm with Joe! Unzip the vinyl and get to the missing flashing and/or moisture barrier! Common problem is the vinyl sits on the shingles and is not one inch above with step flashing behind. Removing the drywall may only find the result and not the cause.

When vinyl siding is used, I never see a gap and the flashings/wrap aren't visible. 

Like so...

You guys are really digging at something that doesn't exist.  If it were roof related, there would be staining somewhere above this point as well and not just at this level.  And staining from water leaking sure wouldn't form multiple straight stains.  I have to agree with the mop theory or something else that was leaned against the wall.

You are entitled to your opinion, just as we are. I see this same moisture staining often enough. Many times it is behind shelving or other such items that would positively eliminate the "wet mop" theory. One difference from your area though... I see heavy snow and thick ice for prolonged periods of time, versus your 'few weeks' at a time (at best), thus we have a much higher propensity for this such problem.

Note: the OP is from Wisconsin, so he shares the same severe Winter weather patterns I do.

Another note: I tend to see this issue more often when the attached garage is not heated, or the heater is rarely utilized, thus opening the discussion to the possibility of a condensation related issue...