oops. no image.

Could be the mycelium of a fungus or tree roots if there are no above ground parts.

Reddish brown and looking like roots sounds a lot like it IS roots!! Tree roots can spread a very great distance away from the parent tree - in some cases as much as the length of a football field. And depending on species, they can form a very dense mat of barely sub soil surface feeder roots that will take advantage of any improved or enriched soils - like your raised beds.

But without a visual to confirm, this is all just speculation. Can you try a photo again? Any large mature trees in the near vicinity?

Where do you live?

Please try again to post the picture.