Originally Posted by OccasionallyFlying

It might've also been the time you arrived from your DOM flight. The sterile US Connections Facility is open at 0700 and closes at 1900 sharp since CATSA stops processing the last US cnx pax at 1900. Before or after these times, you need to exit landside and go through CATSA from E Gate check-in.

Otherwise, the signage points towards D Gates(INTL) and the airside US Cnx Facility during these hours. If you're arriving anytime between 0630-0700, its better to wait in C Gates until the airside US cnx facility opens at 0700 since the connections CATSA/USCBP line is always faster. No priority/NEXUS/GE/TSAPre line at the moment due to the drop in demand from COVID.