The airport was declared "safe" at about 19:00, with police finding what they think might be a CS gas spray.
In a statement, the Metropolitan Police said they are not treating the case as "terrorist related".
The force statement continued: "A search of the airport led to the discovery of what is believed to be a CS gas spray.
"Whilst the cause of the incident has not yet been confirmed, officers are investigating whether it was the result of an accidental discharge of the spray."