A yichud situation at a house with no one else there- does pesach pasuach help, it’s on a very populated street. Also, does it help that a person is coming back to the house and an unannounced time, can that be considered a shomer?
Pesach pasuach helps if the door is left slightly ajar – not if the door is closed but not locked, whereby the chances of somebody simply “walking in off the street” are negligible.
However, the fact that somebody might be coming home unannounced certainly helps, provided that the parties are aware of this concern.
Please see here for details, especially the sections “Pesach Pasuach” and “Giving the Key to Others”.