Unspecified time frame, except that it's post-"The Kiss." (Very mild spoilers for a few other episodes, but not really spoilers since this story changes what happens in those episodes anyway.)
AN: I debated quite a bit about whether to post this one here or not until after the "final episode" ones, because it extends further than post-"The Kiss." I ultimately decided to put it here because the inspiration for it was from Sue arguing that their making out (that Jack insists was not making out) was "part of the undercover." But if you want to come back and read it later, the last actual part-of-the-series reference in it is episode 3.14, "Who Wants to Be a Millionaire." They're all very minor references though.
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The first time it happened was the very first case they worked together. Sue had left the van to get a closer look at the faces of those whose lips she was trying to read, and Jack had run after her. When he caught up with her, he said, "There you are, sweetheart," and kissed her cheek. It was quick, innocent, only the cheek, and all for the sake of making the impromptu cover believable.
It was enough to make them both realize it was a very bad idea, and for nearly two years they managed to avoid any semblance of kisses. Oh, there were many other times people assumed they were a couple, of course, and that one time they went undercover as a married couple, but any form of kisses were a no-go. They were simply too dangerous.
Which is why the next time it happened wasn't until two years later, at Callahan and Merced.
Jack and Sue were doing some late night investigation in Hillary Kern's office when Kern's personal assistant, Janice, came into the office unexpectedly. So they did the only thing they logically could to hide their true reason for being there—they started kissing.
A lot.
Because being caught kissing because they were so excited about the deposition they'd brought was . . . somehow more logical than just being there to bring the deposition. Probably.
The next time they had to go on a "date" as part of a stakeout, they figured they should probably give each other a few kisses while they were out. For the cover, of course.
When Sue was undercover in a cult and she was passing information to Jack, they saw someone coming from the cult who might recognize her. So of course they started making out. To hide her face. For the sake of the cover. Of course.
When they went on a double-date with Randy and his new (Deaf) girlfriend so Sue could help interpret, it may not have technically been an official assignment, but still, they were technically undercover. It made sense that they should share a kiss or two. Or five. Or . . . they might have lost count.
And then only a few days later, when they were following Tara and Horace around while Tara was undercover, they figured they were supposed to be blending in too, so they walked along holding hands, exchanging kisses, and just generally being their thorough selves making sure that their cover was perfectly intact.
When they were caught on the Kiss Cam at a local hockey game where they were keeping an eye on a suspect, they had a hard time convincing the others that it was really for the sake of the cover. But could they help it? They were just blending in.
When they went undercover as two substitute teachers and tried to insist that the teachers being in a relationship was part of the cover and that's why they'd been caught kissing, the director strongly suggested that they find a way to resolve this.
Bobby recalled all of these instances (with varying degrees of embarrassing details) in his Best Man's speech at Sue and Jack's wedding. "So here's to Sue and Jack," he said, raising his champagne, "the only two agents I've ever known who would go so far as to get married . . . ." he winked at them. ". . . for the sake of a cover."