Is this after some time out?
Sessions on your site are set to expire after 1 hour. Admin Zone > Setup > Configuration > Security options > General
You can raise that number.

If it's after you close and reopen a browser window or tab, this would be normal because it has to unconfirm the session as a measure to stop CSRF attacks from malicious sites.

I understand what you say and I will watch closely next time it happens. It just seemed to be occurring frequently but when I work I often have several windows open, close them, open others, do other things in between. I've just tested under normal circumstances e.g. login then try 'Admin' and it was OK. Will let you know.
Rgds Chris

PS I see now that Tracker is where bugs are reported & replied to. I got to this one via the link in your email but discovered I could also locate it Support/ Known bugs/ My View where 3474 is not listed under 'Reported by me', only as 'Recently modified' (which it has just been by me). Would it not be better if the user’s ‘Support tickets’ page links to (in a list) the user’s bug reports & vice versa?
Presumably the 3474 ID number was displayed when I made the report & I didn’t take note of it.

"where 3474 is not listed under 'Reported by me'"

You probably had it filtered to not show closed issues.

An email notification would have been sent out when I replied and closed the issue previously, which would have included a link back to the issue.