Under cross-examination today a senior investigator, Pol Col Cherdpong Chiewpreecha, told a Koh Samui court that CCTV footage from the pier had not been examined after the double murders.
The pier is close to the beach were the battered bodies of the British holidaymakers were found and is the main route to and from the resort island.
"I asked whether police checked CCTV footage. He [the witness] replied no and that police had collected the footage but investigators thought it wasn't relevant," defence lawyer Nakhon Chomphuchat told AFP after the morning session.
The defence also alleged that a small boat was seen leaving the island shortly after the killings but the officer was unable to confirm this information.
Prosecutors have argued that DNA evidence implicates the two Myanmar migrants, but the defence says an under-pressure police force have coerced confessions, later retracted, from the pair.
Attempts by the defence to independently test some of the key forensic evidence against their clients were thwarted after police told an earlier hearing that the samples had been used up.
Police say Mr Miller had been struck by a single blow and left to drown in shallow surf while Ms Witheridge had been raped and then beaten to death with a garden hoe.
Among a litany of apparent mistakes in the hours after the grisly discovery, Thai police were criticised for failing to secure the crime scene or close the pier.