Peah, Chapter 5, Mishnah 7. This mishnah deals with several details relating to the question of what qualifies as shichechah - sheaves forgotten in the field.
The first two cases in the mishnah:
"Ha'omer sheshechachuhu po'alim v'lo shachach ba'al habayis" - a sheaf of grain that the workers forgot about, but the owner did not forget about; he is in the field and is aware that it is there.
"Shachach ba'al habayis v'lo shechachuhu po'alim" - the owner forgot about the sheaf, but the workers did not forget about it.
In both of these cases the mishnah rules that it is not shichechah, since as long as the workers remember it or the owner remembers it there is no forgetting here, because the sheaf was not entirely forgotten.
The requirement of being present in the field:
However, if the owner is not in the field but is in the city, then even though he has not forgotten about the sheaf, that does not help. He must actually be present there at that time.
"Amdu aniyim b'fanav o shechipu b'kash":
Another case: poor people stood in front of what might have become a forgotten sheaf, and by standing there they blocked it from the owner's view; or they covered it with straw so that the owner would not see it and would not remember it. "Harei zeh eino shichechah" - this is not considered shichechah, since in the natural course of things the owner would have noticed the sheaf and would not have forgotten it, and it was only the outside act of the poor people that caused him not to notice it and to forget about it.
In summary: in this mishnah we learned that a sheaf becomes shichechah only when it is forgotten from every side - both by the workers and by the owner who is present in the field; and even forgetting that was brought about by the poor people themselves, by standing in front of the sheaf or covering it with straw, is not considered shichechah.