Do you think this is a proper use of a staff training day? Should we have been asked to approve the closure and given information about what was planned? Is it governors' concern at all? Are we entitled to a formal report?
If the account you have been given was accurate and that was really all that took place, I certainly don't think a staff training day should have been used that way. You describe routine housekeeping tasks which are usually fitted into a normal working schedule.
Training days are strictly for professional development, not necessarily with external guidance but always with some element of extending or improving teachers' skills, discussing policies and problems, or planning together to move the school's work forward. It's possible, I suppose, to talk profitably while doing some undemanding task like cutting up paper - don't we all? Also planning the content of topic work and its place in the curriculum would qualify as a professional development activity, but not just assembling the jam-jars. But it doesn't sound like that.
Strictly speaking, governors should approve day closures and - strictly speaking - they are also responsible for authorising the use to which funds allocated for staff development are put. The DFE recently confirmed this in relation to GEST money - though I would not suggest that they concern themselves with the detail of the professional process. At the same time, I know that many governing bodies never hear professional development mentioned or have any say in the way funds are spent.
Ask if the head's report to you could cover staff development, or ask for an item on every agenda to cover plans for and reports on in-service activity.
I did have the treacherous thought that if your informant had called what they did a "review of teaching materials" you might have been satisfied!