Sleeper Caches are exploration sites featuring environmental dangers and different gameplay compared to other exploration sites. They can be found in all areas of space and you may have to hack structures to disable defenses or unlock some assistance, all while avoiding environmental effects, such as nebulae causing constant damage to your ship. Sleeper Caches are the only locations where blueprints for polarized weapons may be found.
More details about the Sleeper Cache sites can be found in the following Dev Blog:
Sleeper cache exploration sites & polarized weapons