Source and decoding
ותקני ליה שדהו, דאמר רבי יוסי ברבי חנינא: חצרו של אדם קונה לו שלא מדעתו! הני מילי בחצר המשתמרת, אבל חצר שאינה משתמרת, אי עומד בצד שדהו – אין, אי לא – לא.
Why should his field not acquire for him? Rabbi Yosei son of Rabbi Hanina said: A person’s courtyard acquires for him without his knowledge! That applies to a guarded courtyard. In an unguarded courtyard, if he stands beside his field, yes; otherwise, no.
A guarded courtyard acquires without owner awareness. The unguarded field here requires standing beside it. The find matters too: an injured deer differs from a normally running one. Declaration does not replace control.