In case a site is infected, will the TrustSeal display threat notification to the visitors on site?

If a scan fails, site visitors will not be alerted to any problem. The TrustSeal will simply continue to display the last date when all scans were passed. If the site owner fails to rectify the problem, within a few days SiteLock will remove the TrustSeal from the site. The TrustSeal will never indicate that a website has failed a scan.