Thats a good idea, which has already been introduced (no offence, but is has, sorry

) But I read somewhere that the Thieves guild wanted to make a kind of secret guild, with only the highest ranks visible for the public, and for the rest everybody in the shadows.
So the better idea would be if only the members of that guild see, that they belong to the same guild.

And if you might be looking for a certain guild, you would have to go to the nearest building which is the guilds. Though that would mean every guild would have there own house, and a we all know guild come and go, so that could be a problem, abandoned buildings and all