That Q was the magnifying glass that indicates a search bar, and when I thought about it, I realised that it's not actually obvious that that's what it means.
I suppose it's because detectives searching for clues use a magnifying glass (Inspector Gadget, for instance).
Anyway, it's an 'icon'. Not an icon in the computer sense, though it's that too, but an 'iconic sign'. Pictures of things are iconic signs of those things, in the theory of C. S. Peirce, who was Mr Semiotics. A sign is something that 'means' or 'stands for' something else, and an iconic sign can be pretty much anything from a drawing of a cat to a little stick figure indicating which toilets are which. The magnifying glass doesn't exactly mean 'search', but it's got some kind of relation to the idea of searching and we recognise that relation and know what it means. As it turns out, the meaning is a bit opaque if you are new to this computer malarkey.
There are other kinds of sign too: indexical signs are ones that have some real connection to the thing they represent (so it's not just a picture of a pipe, but the smell of pipe tobacco, for instance). A symbol, on the other hand, is purely conventional: it means something, but it has no real or apparent relationship to the thing it means. A word is a perfect example of a symbol: there is no reason at all for pipe to mean what it means. Doesn't look like a pipe, sound like a pipe, act like a pipe, so it isn't a pipe. But conventionally, we use it to stand for the concept of 'pipe'. It just so happens that French people do too, but that's not because the word is particularly pipey.
Almost all words are symbolic - they're simply arbitrary conventions that we've all agreed to use in the way that we do. A few can be argued to be iconic: onomatopoeia is supposed to have some characteristic of the sound it imitates. But most are symbols, with meaning assigned through some mysterious process too long ago for us to know how it happened. Dangit.
Anyhoo, so what about sign languages? A common misconception is that sign languages are just gestures. Clearly that's silly. Take about two minutes to actually look at someone using sign language and you can see it's silly. A sign language is a complex thing, just like a spoken language, and just like spoken language, most of the words are symbolic: arbitrary conventions. However, the thing with sign language is that it's visual, so iconic signs are easier to come up with than they are in spoken language (we are much better at imitating the way something looks than the way it sounds). This means that more signs in sign languages are more iconic than words are in spoken languages. Signs for nationalities can often be amusingly (slightly racistly) iconic.
Signs are likely to become less iconic over time, however, as the language evolves, and the origin of the sign is forgotten. It's a bit like when we get a verb from a name and then forget who the person was: boycott come from this chap, but most people neither know nor care. And that magnifying glass has become more and more stylised over time, and is starting to look less and less like an actual magnifying glass. Now that I know it looks like a Q, I'm going to start calling it that and try and convince people that Q is the symbol for 'search'.