One of the many hidden secrets in Star Wars Episode VII: The Force Awakens is the location of the Pink Astro droid which I finally located after my third viewing. The astro droid can be found rolling behind Poe's X-Wing after they land at the Resistance Base for the first time. It's a brief shot, though you can see the pink droid roll right behind the ship towards the right side of the screen for a few seconds. Not terribly hard to find, but easy to miss since it's not the focus of that shot. This droid was in honor of the 501st Legion founder's daughter Katie as she was sadly diagnosed with terminal brain cancer. The droid appeared in the Clone Wars movie and some of the Clone Wars episodes which are considered canon in the Disney Star Wars universe. This was a nice cameo in the Force Awakens and be sure to watch out for R2-KT when you re-watch the film! Will update the image below with a screenshot once the movie releases, this is just to give a view of what the droid looks like beside R2D2 below.
Update: After a fourth viewing R2-KT can also be found later on in the film towards the end when the entire team returns to the resistance base. The droid can be found in the bottom right corner while crowded by folks though still visible.
Update 2: Since the movie is now released on Blu Ray I have some actual screenshots showing R2-KT in two locations.
Read more about R2-KT here on the Star Wars Wookieepedia