How to verify mold on exterior wall has penetrated to interior
The exterior of my home has discoloration which looks like mold. How do I verify if the mold has penetrated into the space between the exterior and interior walls? This winter there was mold growing on the interior walls facing the exterior portion of the house. I cleaned the mold with a bleach solution and kept the air circulating as best we could. Our winters are damp here in the Pacific NW but didn't expect mold to grow indoors. The interior mold didn't return after the cleaning and have since kept items a good distance from those walls to allow the air to circulate.
Mold can be very nasty stuff, so I would encourage you to address this ex post haste. If you don't want to get destructive, there are tools (like fish-eye cameras) that will allow you to drill a small hole, insert a tube and look around. I don't know where to get them, but they do exist; Google. You may need a dehumidifier, too. Relative humidity should be between 20 and 45%, if I recall correctly.
Is the T-111 siding deteriorated in that spot? If it is, then you may have a problem. If not then scrub it off and see if it reappears. It looks like it only appears in that one spot. If it is being caused by splash back from the surface directly below, you probably should redesign that to stop it from re-occurring.