That is interesting. Did you find any material left from rotting or is it very clean. It could be many things. Wood placed a form to pour concrete that was never poured or maybe they were trying to pour some additional ribs for the foundation.
I actually, to answer a bit your second question too, had a similar problem with water and dug a trench under part of the basement slab to drain the water out. It could have been that too.
Either way, don't do like me, get some professional to come out and give you a expert opinion on how to insulate the foundation.