A can of Raid isn't going to help you here. Seeing something in the viewfinder (hair, dust, blob, insect) and not in your shots is common. DSLRs get dirty.
The most likely places the intruder lives is on the main mirror or on the focus screen, though there are some other possibilities that are more complex and vary a bit with different cameras.
Here's the rub, cleaning either the mirror or focus screen, while it can be done by the user, is not something you generally want to tackle. The mirror is very easily damaged, and damaging it can impact exposure and autofocus. The focus screen is often "shimmed" by Nikon to get it to a precise position.
The good news is that virtually any competent camera repair facility can make fairly quick work of doing the type of cleaning you need. Many camera stores have someone on staff that can do this kind of work.
Personally, I just ignore small dust bunnies like this and once every year or two have my working cameras given a full CLA (clean lube adjustment) by NikonUSA. It's a little pricey (just under US$300 last time I had it done), but the camera comes back like new and with everything reset to manufacturing tolerances.
