Indoor motion sensor placement

[Mr_D]

I've been reading about placing infrared indoor motion sensors for alarm systems. They say to keep them away from windows and HVAC vents and don't aim them at windows. So if you have a room with windows on one wall and a vent on the opposite wall, where the heck do you put it? Do they mean to keep the window out of view, suggesting I'd put it in the corner of the wall with the window, or can the window be in view as long as its across the room?

 

[SouthernYankee]

I have my infrared sensor mounted over the windows pointed down a hall. All my infrared sensors are on outside wall pointed to the interior of the house. All my vents are in the ceiling, but the AC and heaters are turn to very High (AC) or Low (heater) when no one is home. I have not had an infrared false alarm.