Given the circumstances of the information involved, the FBI would have cordoned off city hall until the laptop could be secured. Since they would also have had access to the blueprints, this would include securing the tunnel. Next, given the heightened security surrounding the Mayor, the police officer would not admit a large number of gas company employees.
SA Brock says he will get a search warrant for Philadelphia City Hall. Since it is a government building, the 4th Amendment does not apply so no warrant would be required. The 4th Amendment protects citizens (which includes government employees) from search and seizure by the government, not government buildings from the government. Since many FBI agents have a law degree and all are trained on the US Constitution at Quantico, he would have known this. In reality, given the circumstances, the FBI in cooperation with local officials would stage a deception along the lines of what they eventually did.