Detectives investigating suspected drug offences at an address in Firle Road are appealing for two people who helped officers to come forward.
Police executed a Misuse of Drugs Act warrant to enter the property just before 11am on May 22.
Two men and a woman were arrested on suspicion of supplying drugs. All three have been bailed until October 3 pending further enquiries.
During the operation the woman fled outside but was detained by a plain-clothes officer.
As the officer walked her back, a man and a woman in a small blue hatchback car pointed out a package apparently thrown from a window at the property.
This was recovered and found to contain a large amount of what are thought to be Class A drugs – heroin and crack cocaine – which are being forensically tested.
Detective Constable Steve Shimmons, of Eastbourne CID, said, “Unfortunately, the two people who had spotted the package drove off shortly afterwards.
“We are very keen to speak to them and would ask that they, or indeed anyone else who may have relevant information, get in touch as soon as possible.
“They should phone Sussex Police on 101 or email email@example.com, quoting serial 318 of 22/5.”