The former police officer, 31, has pleaded not guilty to murdering ex-partner and TV presenter Jesse Baird and flight attendant Luke Davies.
The couple were fatally shot with Lamarre-Condon's police pistol in Mr Baird's Paddington home in February 2024.
The former senior constable has also denied breaking into Mr Baird's house and stealing his phone and wallet in August 2023 after their relationship ended for the first time.
A long-time friend of the officer told a 15-member NSW Supreme Court jury on Tuesday about the accused killer's jealousy surrounding Mr Baird.
He told her on multiple occasions about hiding outside Mr Baird's house while his ex was inside, she said.
"I was sent photos through the window of Jesse having intercourse with another man," the woman, known by the pseudonym Elizabeth Kennedy, said.
"The accused told me the other person Jesse was with noticed someone at the window so he got up and ran away."
The chat occurred on the encrypted app Signal, she said.
Lamarre-Condon rang his friend another time in 2023 and opened the conversation with "I've done something crazy, I almost got caught", Ms Kennedy said.
The senior constable said he broke into Mr Baird's house using a key he cut for himself and put a bobby pin in the lock to make it look like it had been picked before trying to steal his ex's phone, the witness said.
"He dropped the phone charger as he was picking up the phone (and) Jesse woke up," the friend said.
"(Mr Baird) said: 'What the f***, get out!' and chased him up the street."
Lamarre-Condon allegedly told the friend he took Mr Baird's wallet to make it look like a robbery but was worried he would have had to assault him if caught in the chase to conceal his identity.
Ms Kennedy, who met the officer as a teenager, began testifying about joining Lamarre-Condon on a trip south of Sydney in the days after the February 2024 murders.
In the back of the rented van lay two surfboard bags containing the bodies of Mr Baird and Mr Davies, jurors have been told.
Lamarre-Condon was bubbly during the February 21 trip but wouldn't answer repeated questions about where they were going, she said.
The pair stopped at Bunnings Goulburn, where Ms Kennedy said she was made to buy an angle grinder.
The Crown alleges the angle grinder was later used by Lamarre-Condon to cut a lock on a gate at Cella Farm at Bungonia.
Ms Kennedy was allegedly left by the gate while Lamarre-Condon drove in and attempted to dispose of the men's bodies in a dam using gym weights and quick-set concrete.
When that failed, he allegedly dumped the bodies outside a Greek monastery 30km away.
Ms Kennedy is expected to give further evidence about the trip when the trial resumes on Wednesday.
Before the bulk of her evidence, Ms Kennedy confirmed she gave an induced statement to police, protecting her testimony from being used against her in court.
"Her evidence ... may tend to prove she was an accessory after the fact to (the alleged murders)," Justice Natalie Adams told the jury.
Ms Kennedy has not been charged with any offences relating to the deaths of Mr Baird and Mr Davies.
She earlier told the jury Lamarre-Condon was obsessive before he met Mr Baird.
"He would send me photos of celebrities and make out he was friends with them when he was actually just stalking them," Ms Kennedy told the court.
But it was once Lamarre-Condon began casually dating Mr Baird the officer's jealousy became evident, she added.
1800 RESPECT (1800 737 732)
Lifeline 13 11 14
Men's Referral Service 1300 766 491
14°C