If a mail was sent with a signature as attachment, the attachment icon isn't shown in the message list even if there is another "real" attachment.
Because the attachment icon and the digital signature icon both share the same column in the message list, a composite icon is used: clip_gpg_signed.xpm This composite icon is shown here. It could be that you are using an icon theme which does not utilise a disctinct icon for this case. Have a look at /Help/Icon Legend to identify the icon used by your current theme, labelled 'Message is signed and has attachment(s)'. I will close this as INVALID, but, in light of the above, you still don't see the correct icon, please re-open it and either attach an example message source to this bug item, or, better, send the message to me, privately, using 'forward as attachment'.