The High Court in Glasgow heard the pistol and revolver were found by council workmen who were clearing shrubbery beds behind Coatbridge Library on January 26, 2010.
Ross Monaghan, 30, denies murdering Mr Carroll by gunning him down at the Asda car park in Robroyston, Glasgow, on January 13, 2010.
Patrick McAuley, 48, a gardener with North Lanarkshire Council, told advocate depute Iain McSporran, prosecuting, that he found a plastic bag under a pile of bricks.
Mr McAuley said: "At first I just took it for another piece of rubbish, but when I took it out it had extra weight. I shook the bags and two bags fell out."
The court heard that each of these bags contained a gun.
The jury was told the guns were taken out of the plastic bags and handed around by all six of the workmen at the site.
Mr McAuley's colleague Alexander Wilson, 42, said; "Everybody had a wee look and everybody had a hold of them. They were passed about quite freely. I'd never seen a gun before."
Mr McSporran asked Mr Wilson: "Did anyone interfere with the guns?" and he replied: "Yes, one of the guys had the revolver and touched it – the bullets dropped out on the ground. He proceeded to pick them back up."
Mr Wilson said he did not have a mobile phone with him, so he went into Coatbridge Library and phoned the police.
Under cross-examination by defence QC Derek Ogg, Mr Wilson said police left behind one of the bags in which the guns had been wrapped, and that he handed it in to them two days later.
Mr Ogg said: "The police did not collect this bag along with the guns and other plastic bags, but you realised the significance of it when you went back two days later and handed it in to the police?" Mr Wilson replied: "It's not up to me to tell the police how to do their job. That bag sat on my tractor for two days. I said, 'Will I hand it in or not?' and then said 'I'd better'."
Detective Constable Graeme Begley, 37, said in evidence that he went to Academy Street, Coatbridge, at around 11am and was shown a plastic bag. He said he could see a revolver sticking out of it. DC Begley said he did not touch anything and arranged for a firearms officer to examine the guns and for a police photographer to attend.
Cross-examining, Mr Ogg asked DC Begley "There are no photographs taken of the actual spot where the guns were found?" to which he replied: "I would concede that."
Mr Ogg then said: "Is that not a bit sloppy?" and DC Begley replied: "I wouldn't like to comment."
Judge Lord Brailsford asked: "Would it be normal practice to photograph the place where items were found," and DC Begley said: "It would be, yes."
The defence QC then said: "These guns had been taken out of the bags they were wrapped in, handed round six workmen – some may have been wearing gloves and some not – from a DNA contamination point of view that's quite horrific, is it not?." DC Begley replied: "Yes, I agree."
The jury heard that prosecution and defence agree the guns found in Coatbridge were the ones used to shoot Mr Carroll.
It is alleged that while masked and acting with others, Monaghan murdered 29-year-old Mr Carroll by repeatedly discharged loaded handguns at him.
He denies all of the charges against him and has lodged a special defence of incrimination against eight people.
The trial before judge Lord Brailsford continues.