I can believe that there may be "things" flying out of radar, just happening with no rational explanation, but we also know that amphetamines are prone to cause several delusion, so, meta-amphetamine meaning 'beyond amph" it also means that meth is very likely to cause states that could be labelled as meta- delusion, something beyond paranoia.
Never did meth methmyself, but used to bing heavily on IV cocaine and sure I have seen shadow people and weird shit, but I think all those demons and evil entities are related to deeply hiden fears that we have and when a mind gets overstimulated, it tends to release some of those fears on us.
Have you noticed that very often the shadow people you see doesn't belong to your own race, religion or social- economic level?