For their own records, the board should take minutes during executive session. However, because the minutes reflect motions taken during an executive session, it is not required by Robert's Rules that those minutes are to be published and available to those not present during executive session.
We always took minutes during our board meetings. No one ever asked to see them, but if they had, I wouldn't have had any problem with that. Maybe no one ever asked because our board meetings were open if someone wanted to attend-they couldn't comment or ask questions, but they could listen.
Do you feel like minutes aren't being taken because of convenience or because they are hiding things? For that matter, unless you are there, how do you know those minutes are accurate?
Our exec board officially meets once a month, the week before the general meeting. They don't take any minutes during these meetings. Should they be taking minutes and should they be made available if requested?