Hi, bfluet, thanks for replying.
The member of staff uses Preview mode to view mail.
They have their iPhone with them, switched on, at all times. I use an iPod touch and can see that when the mail application is started it automatically polls all mail accounts for new mail. So, when the member of staff wants to check their hotmail account, for example, from work, I assume that the application on the iPhone behaves in the same way and by default checks all mail accounts. This would create the LCK file.
The issue of course, is that the LCK file is not removed in a timely fashion. Unfortunately, as I said above, the creation of the LCK file is not always guaranteed. It would be interesting to see if anyone else has noticed this behaviour. I will explore Apple's forums and see if this issue has been reported before.
In the meantime, if anyone else has any experience of this, please share.