Yes, this is another lazy post, i. e. me linking to someone else’s work. In addition to my post of September 4th, I found another list of off-mainstream films of 2015: Top 10 Movies You Probably Missed in 2015
With the possible exception of Tangerine, I do not believe that any of the films on that list went “under the radar”, or qualify as “hidden” in any other way. But some of them left the public consciousness much quicker than they should have. Usually that happens because other, bigger films are being released or other news push them off the agenda.
The list made by filmslob here on wordpress contains (IMO) three such films: Dope, The End of the Tour, and Love & Mercy. I have not seen these films, but I remember that they all received very good reviews on podcasts and the like when they came out, but then dropped out of public conversation within a week or two. All of them have excellent scores on Rotten Tomatoes, and if – like me – you have not yet had the chance to see them, now may be the time. The prices for used DVDs for these three films have come way down, and apparently in the US both The End of the Tour and Love & Mercy can currently be streamed for free on amazon.com if you have an Amazon Prime membership.