I have the URL set to the background color so it doesn't show in a 728x90 leaderboard, and I have noticed over the years that sometimes google shows the url and sometimes it doesn't. Any logic to this?
That's probably against the TOS. Advertisers pay to have their URL displayed in ads so that people can associate product with site.
is it an image ad? if so than some image ads do not have urls by default. if it is a text ad than that is strange indeed as the adsense code will not allow it and therefore should be showing all the time. and, yes this is against the TOS.
it was a text ad.. i have noticed lately that the text link color will change even if you try to blend it in.. but months back i would notice it would be gone completely.... resulting in higher CTR.
it is not allowed.a while ago they change it so if the colors are too close, the url color will change so it does show.