An red alert alarm will be sounding all over the lobby area along with a computer voice saying: "Warning security lockdown! The laboratory bulkheads are closing! Please stand clear!". To use the main entrance you need to get down the stairs and to the front doors before 3 rounds have passed or the security gate will have fully closed. A blast door will then close behind it so don't think you can just smash the gate down.
If you fail to make it to the entrance before the blast doors close, then you will have to take a long detour through the service areas which will have a lot more security. Regardless of which route you take, you will still end up in the laboratory areas.