Anything that seems suspicious to you. Trust your instincts. Then call us.

Terrorists visit and research potential targets. We call this activity hostile reconnaissance. If you suspect you are witnessing

• hostile reconnaissance
• a suspicious vehicle
• an unattended bag
• any other suspicious activity
  Telephone the police without delay.

What is suspicious? - Filming, drawing, taking notes or photographs, or just watching for extended periods, focusing on security cameras, hallways, fire exits, access and egress routes.

Source: https://www.cityoflondon.police.uk/advice-and-support/countering-terrorism/Pages/report-suspicious-activity.aspx