because a beam implies it's going in a single direction say, like a laser beam
You have to be inline with the beam to see / detect it
If you have ever experimented with lasers you will know that if there is dust/smoke etc in the air that the laser
is shining through then you will see the beam off to the side as a small amount of the light is reflected out to the sides
Most of these have a lens built into the top of the part. That focuses the IR on to the chip surface for detection. Think of it like a old bulb type flash light in reverse. You need to aim that lens at the IR source your trying to detect. Or it can be used at the focal point of a parabolic reflector for added range. sensor pointed at the reflector.
Important question! I used to work on the old F-102A plane while in the Air Force. It used an IR receiver made this way. Also nitrogen cooled for more sensitivity. 15 or more miles detection on a small airplane's engine as I recall tracking them while testing the system.