Opening one email marks 4 as read

I have had three users report that when they open a single email two or three emails on either side of the one opened get marked as read (ie turn black) when they return to their inbox, even though they have not read those emails.

These users only have their mail file on the server no replicas. Two of the three the problem seemed to be fixed by running a compact -D but the third is still having the problem. It is intermittent, and I have not observed it myself. A fourth user had the problem that deleting a single document actually moved 4 documents to the trash, again intermittent This one also fixed by a compact -D. All using 6.5.4 FP3

Has anyone else had this problem and if so do you have a fix?

Tony.