It's been going on for somewhat longer than that, and isn't directly to do with the membership numbers, nor with the £107k loan from Murrell to the SNP that Sturgeon didn't know about.
This is a reasonable primer from last year.
https://www.scottishdailyexpress.co.uk/news/politics/files-snps-missing-600k-finally-28934498
I note that the Express always write "missing" rather than missing, which implies that this is speculation and if they claimed it as fact they'd get the hell sued out of them.
The other thing I've taken from the article is that it isn't clear whether the money has vanished (which would be the implication of "missing' or certainly missing), or has been spent on purposes that aren't related to the purpose for which the funds were raised - specifically "on a campaign for a second referendum". I guess the challenge here would be to determine whether monies raised in such a manner were 'restricted' or 'unrestricted' in terms of their use and if 'restricted' the limits for their use. I would image that the SNP, as a party that supports independence which can only be achieved through a referendum could claim that any campaigning activity is, at least indirectly, aimed at achieving a second referendum. Whether that would be deemed legitimate would be, I guess, for the courts to decide.
But coming back to the original question - is this money literally missing, in other words no-one knows what has happened to it, or is there an audit trail where it was received and has been spent, but arguably not on the purposes for which it was raised.
I think the bigger picture here is that a little over a decade ago the SNP was a small campaigning operation which probably has processes and systems in place suitable for an organisation with a membership of about 10,000. Then suddenly it increases in size 10 fold and failed to put in place processes and systems that work for a much larger organisation and continued to operate in a cottage industry manner as if it remained a 10,000 member organisation.
