Anything that Homeland Security has deemed as a credible threat to public safety, is categorized as a Homeland Security Check. This could change daily. The officers have to log on their daily activity sheets, the time and the Event Number that was issued to them when the check is complete.
Most of the sites deemed worthy of DHS checks aren't really suprising. hese are your major bridges, train stations, bus terminals and such: The so-called high-value targets.