Normal cell diffusion in normal organism?
In a healthy body, cells travel. 
But this is very dependent of the cell type. Some cell type will travel in all the body (lymphocytes, plasmocytes, etc) while other do not have any migration capability.

The concept of diffusion is wrong to apply here, indeed the scales at which you are looking are not molecular. 

Moreover, there are very active mechanism to keep cell type in place, for example, a concept called preferential attachment will maintain the cells grouped by cell type. If you take two balls A and B and A attached more to A and B to B, and you shake, you will see appearance of clusters of A and clusters of B. But that is only one such mechanism and a lot of other take play here. 

see this: http://www.ncbi.nlm.nih.gov/books/NBK26937/

Thus, the probability of finding a cell type NOT at its normal position (that is one that does not travel) is maybe not zero, but so close to it that you will not find a cell outside of their normal position. 

I hope this answers your question ?

Reference
Cooper GM. The Cell: A Molecular Approach. 2nd ed. Sunderland (MA): Sinauer Associates; 2000