We cannot tell where an electron is exactly. They orbit around the atom but the orbital radius is not definite. However, you are able to determine where they are most likely to appear with the help of Schrodinger's equation.
Even so, the exact position of other particles, such as protons and neutrons can be determined. Since our body is made up of more than just electrons, it is possible to determined their position.
If you want to dig deeper, we can talk about relativity which brings up the question on which frame of reference we are comparing with. Different reference frame will describe the object differently; whether it is the position, shape, direction of motion, etc.
I am not so sure about matter popping in and out of existence but I do know that if you transport matter at an impossibly high speed, you can convert matter into waves such as radio waves and light. This is one way you can make matter disappear. Or you could try converting it into energy which according to Einstein's equation, is possible.