A vehicle was also seized in Belfast
The PSNI say a viable device discovered in Lisburn may have fallen from a vehicle.
A security alert took place in the Market Place area of the city yesterday afternoon, where the device was discovered.
Police are now appealing for information.
Superintendent Brian Kee said: "Our investigation into the circumstances surrounding this security alert is at a very early stage."
"We are following a number of lines of enquiry, one of these is that the device may have fallen from a vehicle."
As part of a follow up operation, a vehicle was seized in the Lagmore area of Belfast last night.
Police want to hear from witnesses or anyone who was in the Market Place area of Lisburn yesterday and who may have information that could assist the investigation.