Strange muck in quilt veneer?

I'm finishing a quilted maple veneer with thinned boiled linseed oil (2 coats in so far) with some sanding in between layers. So far, it's looking good. Except when it's not.

There seems to be small holes/chips/dents/lower grain in one area that attracts this weird, greyish muck.

Attachment 38923 Attachment 38924

it's very hard to capture but it's clear when the body is in front of you. The anomaly in question in the left picture is a strip in the middle (middle going vertically, opening the picture makes it clear) and in the right photo it's pretty central.

Any advice on cleaning this? sand it out? Tiny bit of thinners?