Learn English – it called when people/society expect you to be something because of your gender/nationality

Based on my own experience, I can report that an extended "ooooo" sound is used in this way.¹

It is used in other ways, too, but in most cases it is an expression of mild mocking, as if the person making the noise is pretending to be scandalized by the possibility of the two people being together in one way or another.

There's a tone of excitement, but also maybe some judgement. A lot of all of this will depend on context, including the relationship of the speaker to the person or people being teased, audience present, tone, and facial expression. Excitement might be indicated by a shorter sound with raised pitch in the middle, while mock shock might be expressed with a longer "oooo" that either goes up in pitch at the end or builds steadily higher throughout. This last possibility can be generalized to any time someone is suggesting that someone has been "found out" whether they are being discovered to be an "item" or misbehaved in some mild way.

Physical gestures that might accompany this sound include nudging, eyebrow raises, etc.

Children might do this sort of teasing with the K-I-S-S-I-N-G song, and adults might also just to be silly.

[Name] and [Name]
sitting in a tree,
K-I-S-S-I-N-G.
First comes love,
then comes marriage,
then comes baby
in a baby carriage!

_{1. I attempted to substantiate this part of my answer with website references, but online search for "ooo", "teasing", and/or "couple" has proven to be quite the NSFW minefield and I am currently at work.}

It's good-natured humor. The signs are designed to be amusing.

Smart business owners do not purposefully insult their customer base, so we can cross insulting and disdainful off your list:

Sarcastic, insulting, humorous, witty, disdainful?

Oftentimes, the signs allude to certain cultural references, which is why the full extent of the humor might be lost on the learner. For example, there is a famous bumper sticker that says:

This has been around for decades, and it was designed to be a witty way of discouraging people from tailgating you on the road (i.e., from following you too closely). After a while, though, these bumper stickers became more commonplace. As is the case with many witticisms, the joke got stale as it became more ubiquitous. So, variations started to appear:

So, in the case of the sign at the restaurant:

If you are reading this, it means you are literate. Throw waste plates in a dustbin.

the first part of that is just another spinoff of the dozens of bumper stickers that begin with IF YOU CAN READ THIS.

As for the surveillance sign, in the early days of television, there was a television show called Candid Camera, where people would play stunts on unsuspecting victims. When it was time to let the person know they were part of a gag, they would be told the show's famous tagline, "Smile! You're on Candid Camera." Many people in the U.S. are familiar with the tagline, even if they've rarely or never watched the show. So the surveillance sign is playing off that television show and its famous line when the truth is revealed.

If someone is not familiar with these cultural references, then the humor of the variants could easily be lost.