GMail scans for viruses after we open the email?

This seems a bit odd, I send an email with GMail, I go into my Sent folder 5 hours later and open the email. Then GMail says "Scanning for viruses ..." before I can download the attachments... surely it should just scan and store a flag that it has been checked? Instead I'm kept waiting 5 secs per attachments before I can download them!