Other than finding it on Google Maps, it certainly seems to be a secret. Once you arrive at the car park, there is really nothing to indicate the Lagoon is here, just a white building. Enter that, and voila !
secret-lagoon
Entry fee was about the same as the basic at Blue Lagoon, and towels could be hired. You take shoes off and leave them in an anti room, then have a shower (naked) and get thoroughly clean, leaving your stuff in a free locker.
Exit to the lagoon from the showers is fairly close to the stairs into the lagoon. Not really a need for any footwear.
The waters are thermal, seemingly feed by a small bubbling geyser off to the back left. Not the silica style of Blue Lagoon or Myvatn Nature Baths, but very warm just the same; with that hint of volcanic/sulfur smell. The smell is noticeable, but not overwhelming at all. Get around Iceland enough, and you will be used to it in many places.
