Just an update .....
I went to a specialist Balsa board producer this morning . Has a great rep. Very honest guy.
He had a look at this and another soft spot that has been near rail for three years (which I have covered with two layers of rail tape. btw Bert saw pics of this and "endorsed" and I have had no issues since) . He told me what he could do to repair but suggested that I do not do anything for now except put a large piece of ding tape over .
He didn't not think discoloration was due to water ingress because it went long length of board .
To fix probably would need to open and dry out for month (if there was any water in there in the first place which we wasn't convinced there was ) and put in a new "plank" of balsa . Very honest man and glad I got to meet him
BTW soft he reckons is from knee / rock / other hitting it . Similar to what i have done to front . Here is a pic of front "bruise". No issues for three years since using rail tape approach