When you hold your mouse over people in lists for a few seconds, addresses, availability, preferences, and certification hints appear. This tool allows you to define which background checks will be displayed and which will show alerts for expired or missing certifications. Open this tool from within the Certifications setup tool by clicking on Popup hints.
For each type of person, in the first column select from the drop-down menu which background checks and certifications to display. In the second column select any background checks or certifications for which you want alerts. If any certifications of these types are either not present or are expired, an alert will show in red as part of the hint for that person.