At least this particular abomination is something you would know about, and presumably would have allowed it to be installed in your body.
Hitachi has developed an RFID nanochip about 1/25 the size of a grain of table salt, which would be easily injectable along with a ‘vaccine’ with a 21 gauge or larger injection needle.
That's about the limit as far as size goes for anything active that would be available to the public. Get smaller and you are talking mainly passive stuff that can spread in the vascular system and wind up embedding in various tissues.
What hinders the engineering of powerful, active embedded objects is electron tunneling... you can't go far below the 2nm limit... but that is awfully small, you obviously cannot see a 2nm object using visible light..and you cannot get smaller unless you go to carbon nanotubes and even then only to slightly below 1nm.
Diminutive, passive objects must be manipulated using an external device. Only simple I/O is possible using various techniques that manipulate the objects mainly using electromagnetic pulses with multiple frequencies, magnitudes, vectors. These tiny objects are electrically conductive and have other properties that facilitate I/O.
If multiple objects become embedded and fixed in tissue they produce a distinct pattern akin to a fingerprint. If you know where all such objects are within a piece of tissue then external manipulation is easier since you have a roadmap.. each random pattern of embedded objects will be very different.
Light of various frequencies can also be used to interact with such small objects as it can penetrate the skull and reach the objects since the skull is semi-transparent to light.
Ultrasonic sound can also reach very tiny embedded objects but I know little of that tech.
To interact with extremely tiny embedded objects you have the choice of.
-Magnetic pulses
-Directly applied dc or ac, generally at the skins surface
-Light
-Ultrasonic sound
-chemical manipulation
-changes in pressure/piezo effects
-interaction with active objects
Or combinations of the methods.
The last, interaction with active objects, is again limited due to the absolute size limit of roughly 1-2nm. It is at the extreme limit of current technology. Any such device will have limited capabilities due to the small size available for the design