Typical shingled gable roof approximately 10 years old. Found what looks like tar leaking through some sheathing seams. The roof surface appears normal. Any input would be appreciated.
I doubt it came from the shingles, John. My guess is that the installer (probably a homeowner) wanted to seal the joints between roof sheathing panels against leaks, so he applied mastic to the joints as he went. Not a good idea, since it will limit the ability of the panels to expand with changes in moisture content. There’s the potential for bowing (bulging) of the sheathing as panels expand.
The potential problems represented by that condition are related to the level of seasonal changes in humidity in the area in which the home is built. Maryland is pretty different from Colorado in that respect so you’re a better judge than me.
I’m with Kenton. Based on the pictures 2 and 3, although a little difficult to tell, it looks like the sheathing was going bad, probably from the inside, attic. They may have applied tar (maybe hot tar) to the entire roof to seal it.