Odd phenomena on the inside of the back off concert ukulele

Hello, so I picked up this solid Hawaiian koa ukulele on ebay. Then a few weeks pass and i find a tiny crack in the back about one 1" of length. Then I look inside and the crack is more obvious. It doesn´t seem to pass below the lower bout bar. But at the end of the crack by the bar, there is like missing piece of the back (triangular in shape).
When I swept a gently moisturized tops over the area it got strongly brownish coloured. When I swept a gently moisturized tops over another part of the back it didn´t. So it is like some fine grains of dust just in the vicinity of the crack.
Can anybody please help me to tell what this might be?

To clarify, Hennig, did you sweep the Q-tip on the inside of the uke (i.e., the location shown in the photo), or on the corresponding area on the outside of the uke?

Bill Sheehan said:

To clarify, Hennig, did you sweep the Q-tip on the inside of the uke (i.e., the location shown in the photo), or on the corresponding area on the outside of the uke?

Click to expand...

On the inside, yes

Henning

Is there a well repaired vertical crack at right angles to that horizontal one? The horizontal cracks I have seen are by the neck block, probably been sat on, and with associated verbal cracks.
What really counts is how it plays and sounds, only you know it is there.

@PetalumaRescuke There is no other crack. It plays and sounds surprisingly well.

Has nobody so far experienced this before?
Thanks in advance
regards

This is just a WAG, but it may have been a filler used by the maker to disguise the fact that there was a crack. Often wood dust mixed with a medium to hold it in place.

I had some fall out of my Mango tenor recently, but I couldn't find a crack or anything where I could see one.