New ridge vent was just installed and I noticed 2 shingles across my ridge that have these tiny square patches on them. I'm not entirely sure what they are, I'm guessing it is covering a nail or screw.
There's always a last shingle as the caps overlap. If you have two different ones this way, there were two "last shingles" (not a huge deal.)
If you put another shingle on top, you'd have to nail that one down, and then you'd be back to what these appear to be - glued on bits of shingle to cover the nailheads.
I can show you plenty of roofs where the last cap shingle nails are exposed, perhaps with a dab of roofing cement. You got the fancier version of that.
"...But you could just glue on a whole shingle," you say! The wind would enjoy that, and then you wouldn't have it any more. These little bits will probably stay glued down.