A virus cannot hide its code in the buffers of modems or hard disks, because these memory areas are very small and unprotected - in reality the virus code would be overwritten almost immediately. In any case, part of the viral code would have to be stored in normal DOS memory in order for a virus to function. PC computers execute code that is located in their core memory, and that code only.
It is possible to hide part of the viral code in the memory of a VGA card. Some viruses (like Starship and GoldBug) do so, but even in this case the virus can be found by normal means.
The text was apparently a practical joke that spread uncommonly far. On the other hand, this joke inspired the development of several new viruses. As rumors of PROTO-T spread, some individuals decided to take advantage of its reputation and wrote viruses that contained the text "PROTO-T". Naturally enough, these viruses contained none of the characteristics mentioned in the original description.
The 'real' Proto-T viruses are not known to be in the wild. Their characteristics differ a lot from each other.