Yellow Smiley Face (first appeared in The Stand)

A yellow smiley face seems to pop up in a lot of places where bad things happen. It's most visible as a plot element in Mr Mercedes, where it's the literal calling card of the killer, but it first appears in The Stand on a button pinned to Randall Flagg's jacket.

Yellow Smiley Face is mentioned in the following books:

Desperation (Sep 1996)

Peter's sister Dierdre puts a smiley face sticker on everything, including the bag of weed that gets Peter and Mary arrested.

The big cop gave him a look, those bright gray eyes still peculiarly empty, then reached into the spare-tire well and brought out a Baggie, a big one, a gallon-size, and stuffed full of greenish-brown herbal matter. The flap had been sealed with strapping tape. Plastered on the front was a round yellow sticker. Mr. Smiley-Smile.
- Part I / Chapter 1 / Section 3

Mr Mercedes (Jun 2014)

The Mercedes Killer's signature (which he leaves in the car, in his notes, etc) is a yellow smiley face sticker.

The last thing appears to be an actual letter—a fairly thick one, by the feel—in a business-sized envelope. It is addressed to Det. K. William Hodges (Ret.) at 63 Harper Road. There is no return address. In the upper lefthand corner, where one usually goes, is his second smile-face of the day’s mail delivery. Only this one’s not the winking Walmart Rollback Smiley but rather the email emoticon of Smiley wearing dark glasses and showing his teeth.
- Part 1: Det.-Ret. / Chapter 2

The Regulators (Sep 1996)

Peter Jackson wears a t-shirt with Mr. Smiley-Smile on it while talking with Cary Ripton.

'Yo, Gary!' Peter Jackson says, pulling up beside him. He's wearing faded jeans and a tee-shirt with a big yellow smile-face on it. HAVE A NICE DAY! Mr Smiley-Smile is saying. 'How's it going, bad boy?'
- Chapter 1