You can't. Too many people need to be hushed up.
Every doctor and dentist will notice they suddenly have no patients. Every hospital and nursing home will see their current patients magically get better. Health insurance companies will see claims plummet. Drug and medical equipment companies will see orders shrivel to nothing.
Anyone doing a blood test will notice the nanobots directly or indirectly. They could see the foreign substances of the nanobots themselves in the results, or just the dramatic and unexplained improvements. With a powerful enough microscope, they'll see the nanobots directly.
If all that is somehow hushed up, there's the money trail. If the health care industry suddenly loses most of their business, accountants and economists will notice. There will be ripple effects through the economy. Who keeps paying all the medical workers when there's no work? That's 19 million people in the US alone.
And then there's the simple problem of people recovering from what should have been a fatal or crippling accident. How do you cover that up?
Even if you could cover this up, what you're asking about is security through obscurity and it doesn't work. Trying to cover up the nanobots will probably make it more likely someone will be able to hack them for nefarious purposes and there will be no defense.
Law enforcement and investigation agencies cannot coordinate with each other. Security researchers cannot analyze the nanobots for security vulnerabilities. If they do and discover a vulnerability, who do they report it to? Do they risk being hushed up if they do? If a strange phenomenon starts happening, local agencies won't know to look for possible rogue nanobots allow it to get out of control.
Instead you'd coordinate with law enforcement and security researchers to harden your nanobots against attack, and prepare ways to discover and disarm rogue bots. Security "white hats" would attempt to break the nanobot security and report any possible vulnerabilities to be fixed before they're exploited. Law enforcement and local officials would know how to look for exploited nanobots and test regularly for them.
This is similar what is done to ensure encryption remains unbreakable, or to know when it's become vulnerable.