The Tur wrote: A kohen who has a firstborn animal, and he and all his household members stood with him outside, and the firstborn entered a whole house alone and emerged with a defect, it is permitted to slaughter it; and we are not concerned that perhaps one of the household members hid there and inflicted a defect upon it, or perhaps he dug a pit to cause it to stumble so that it would fall. Thus far his words. And they raised a difficulty against him, for in the Gemara (35b) this law exists according to the one who holds that his household members are not trusted, and it comes to teach us that in such a case it is permitted since they were not in the house, see there. But according to that which we rule that they are trusted, this law is superfluous, for even while they are in the house they are trusted, and all the more so in such a case. And for this reason, the Rambam and the Shulchan Aruch did not bring this law (Beit Yosef). And the commentators labored to answer that in this there is a novel point even for us, and each explained according to his way (Bach, Perishah, and Taz section 2), but there is no source for this. Furthermore, if so, why did the Rambam not write this? And it appears to me that there is a source in the Gemara there, for it asks there: since all the household members are standing outside, what does it come to teach us? And it answers: lest you say we should concern ourselves with suspicion, it comes to teach us [otherwise]. And the Tosafot explained that they would suspect perhaps one of the household members remained in the house; it comes to teach us that we are not concerned. And according to this, for us there is no novel point since they are trusted. But Rashi explained, and this is his wording: since there was no man in the house, shall we concern ourselves that perhaps they caused it from the beginning, etc. Thus far his words. That is to say, that this itself is like an artifice, that they all went outside, and this is not the way of a householder; and it comes to teach us that we are not concerned for this artifice. And according to this, even for us there is a novel point, as is understood. And the Rambam explained like the Tosafot, and therefore did not bring this law, and the Tur explains like Rashi, and therefore found it necessary to bring this law (and see the Rosh there, who also wrote that we are not concerned perhaps one hid in the house, and did not write like the wording of the Tosafot "perhaps one remained in the house," for this hiding is the artifice; and delve into this).
כתב הטור כהן שיש לו בכור ועמד היא וכל בני ביתו אצלו בחוץ ונכנס הבכור לבדו לבית שלם ויצא בעל מום מותר לשוחטו ולא חיישינן שמא אחד מבני הבית נטמן שם והטיל בו מום או שמא חפר גומא להכשילו שיפול בו עכ"ל והקשו עליו דבגמ' (ל"ה:) יש דין זה למאן דס"ל דבני ביתו אין נאמנין וקמ"ל דבכה"ג מותר כיון שלא היו בבית ע"ש אבל למאי דקיי"ל דנאמנים דין זה אך למותר דאף בהיותם בבית נאמנים וכ"ש בכה"ג ומטעם זה לא הביאו הרמב"ם והש"ע דין זה (ב"י) וטרחו המפרשים לתרץ דבזה יש רבותא גם לדידן ופירשו כל אחד לפי דרכו (ב"ח ופרישה וט"ז סק"ב) ואין שום מקור לזה ועוד דא"כ למה לא כתב זה הרמב"ם ולי נראה דיש מקור בגמ' שם דפריך שם כיון שכל ב"ב עומדים בחוץ מאי קמ"ל ומתרץ מהו דתימא ניחוש לחשדא קמ"ל ופי' התוספת שיחשדו שמא נשאר אחד מבני הבית בבית קמ"ל דלא חיישינן ולפ"ז לדידן ליכא רבותא כיון דנאמנים אבל רש"י פי' וז"ל הואיל ולא היה איש בבית ניחוש שמא הם גרמו מתחלה וכו' עכ"ל כלומר דזהו גופה הוה כהערמה שיצאו כולם לחוץ ואין דרך בע"ב כן וקמ"ל דלא חיישינן להערמה זו ולפ"ז גם לדידן יש רבותא כמובן והרמב"ם פי' כהתוס' ולכן לא הביא דין זה והטור מפרש כרש"י ולכן הוצרך להביא דין זה (וערא"ש שם שכתב ג"כ דלא חיישינן שמא נטמן אחד בבית ולא כתב כלשון התוס' שמא נשאר אחד בבית דהטמנה זו היא ההערמה ודו"ק).