Google is doing some A/B testing where in their main site,[0] it will show a slightly modified version of the 2015 logo, as shown in the following Wikipedia article.[1]

You can try this for yourself by opening a private/incognito window, then visiting the main Google search home page.[0] Close and reopen it again until the logo changes.

[0]: https://www.google.com/

[1]: https://en.wikipedia.org/wiki/Google_logo