McIlroy incurred a two-shot penalty on day three for not taking 'full relief' from a spectator crossing point, his lack of understanding of the Rules of Golf coming back to haunt him again as he missed out by one stroke to his playing partner and friend Pablo Larrazabal of Spain.
Renwick, who has enjoyed success in majors with Vijay Singh and caddied for the Australian Steve Elkington when he beat Colin Montgomerie in a play-off for the 1995 PGA Championship, left a note in McIlroy's locker explaining his dismay at having raised the matter.
"If I hadn't said anything and Rory won the tournament by a shot, that wouldn't have been right and I couldn't have lived with myself," said Renwick. "I feel I did the right thing and, if I could have stopped him before he hit the shot, I would have but I was fully 40 yards away at the time. I said to my player Ricardo [Gonzalez] as we walked to the next tee, 'I'm sure he was standing on the walkway, not by much but enough'.
"I couldn't have gone to sleep last night knowing that I hadn't said anything. That's why I put a letter, a nice short one, on his locker today, saying that I'm sure he would appreciate what I did was in good faith. It was pleasing to hear that he'd said there was no animosity; we've all got to adhere to the rules out here."
Larrazabal shot a final-day 67 for a 14-under-par total and McIlroy a 68 to share second place on 13-under with Phil Mickelson, who was also left lamenting having to call a penalty on himself after a 'double hit' playing out of bushes at 13.
"I can't describe how frustrating it is, feeling like I should be standing here at 15-under par for the tournament and winning by one," said McIlroy. "But that's the way it goes. I played the fewest shots of anyone this week. So, I count it as a moral victory more than anything else.
"It is a very positive start to the season so I'm not to let one little negative ruin that. I'm driving the ball well, hitting the ball solidly and giving myself plenty of chances for birdies. And if I can keep doing that, the wins will come. It was only after I finished one behind on the 18th that it started to dawn on me [that] if what happened yesterday had not happened, I would have won. But I ended up not getting a trophy. Pablo played well, and I can't take anything away from him."
It was the second time in three years that a rule infringement has cost McIlroy success in this tournament. Two years ago, Luke Donald picked him up on a rules infringement during the final round that meant McIlroy lost out by a stroke to England's Robert Rock.
Craig Lee, who played the last round in the company of Mickelson, was also left ruing his attempt at a first Tour victory as his two-shot lead going into the final round dissolved into a share of 10th place after a 77 - the joint-highest score of the day.
"I was nervous heading out but excited too; I was feeling upbeat," said the Scot. "I still felt relatively confident after the start but, following the double [bogey] at 11, it was curtains for me. I hit a great shot out of the semi-rough that came up about three inches short, falling back into a bunker, from where I thinned it and then three-putted.
"It was just one of those days. It was great playing with Phil and watching him and his demeanour. Even after he ran up his seven he bounced back with a couple of birdies and that's the sign of a good player. I'm starting to feel a lot more comfortable out there. It's just a case of trying to keep putting myself in that position then hopefully it won't be too long before I can get across the line."
Despite his disappointment, Lee has moved to 22nd and is the highest-placed Scot in the Race to Dubai.
Mickelson had gone from two shots behind the Scot to one ahead with three birdies in 10 holes before arriving at his unlucky 13th.
"It never crossed my mind it might double hit," the left-hander said. "I wanted to use a lob wedge but had to turn around a four iron as I was having trouble seeing the club at address.
"It was just bushes extending all the way back and I wouldn't have had a shot except to go back to the tee box. It p***** me off and I played more aggressively.
"I got refocused and made some birdies and gave myself a chance. If Pablo had not birdied the last hole to win, I would have got into a play-off, so I give him a lot of credit for finishing the tournament off the right way."
Mickelson said he could not remember the last time he suffered a double hit, but added: "I have done it; I have done a lot of crazy s*** over the years."