">>> (c) The real answer to the basis of Christian's question is that indeed
there are many small (Tunguska-type) asteroids passing "close" by the
Earth (that is, passing through cis-lunar space), but the majority of
them are not detected.

If we don't detect them, where do his numbers come from to support the assertion that they exist, at all?"

Well, we have "detect them" in a number of ways. First we have detected larger rocks, we have detected "about" 1/2 of the predicted 1000 rocks larger than 1 km which have orbits which come near Earth's orbit.
http://impact.arc.nasa.gov/index.html
(NASA calls these NEAs [Near Earth Astroids]. NEOs stands for Near Earth Objects which could also include comets or even booster stages from the Apollo program).
The number of rocks increases dramatically the smaller the rock- Tunguska-type is a small rock "around" 50 meters in diameter.
The miltary detects impactors about "the size of a car" hitting Earth in high atmosphere- they release the energy similar to small nuke. There is around 80 of these per year- though don't recall if that is worldwide coverage or if predicted worldwide- but anyways, a lot of them. (Sorry don't have reference handy, but you can look for it somewhere on the web:).
Also counting craters on the Moon (and knowing how "young" the surface is will give a good approximation- for instance if you have lava flow accurately dated).
Also you can get an approximation by percentage of total sky [where these objects could be] which you have scanned and found rocks.
Also the smaller rock are harder to detect but some have been detected- in fact most of the NEAs detected are smaller than 1 km.
There is a "certain" amount of uncertainty due to many factors but it's pretty safe to say that there are more than 10,000 NEAs which are larger than 50 meters (maybe 5-10 times more than this, I think, recall an expert that gave a ballpark of 80,000 [and no, it wasn't Ed:]