The reply from TCMTECH is the most likely to be correct. There is really no other explanation for a dish to be pointed DOWNHILL.
It would be pretty easy to find out, if you haven't already, just wait until you think the person is home and using your wireless. Then turn it off and see if they come out to check the dish. Repeat until satisfied (they might not have noticed the first few times or whatever.
I registered just to reply to this thread. I know the thread is old, but the info I'm adding is even more important today. This thread comes up in a number of search results, so I want anyone reading this to know;
A person with no computer skills could connect to another home that is close, with about 30 minutes and some trial and error. In fact, for short distances, just mounting a dish behind your wireless device pointed at the ap while give surprising results.
Using just a parabolic satellite dish it is possible to connect to wireless ap's that thousands of feet and even miles away. For example, using 2 dishes pointed at each other, tests have achieved a full 11mb bandwidth connection on 802.11b wireless over a distance of 22 miles across a desert.
I myself, using just only old DirecTv dish, a wireless dongle, a coffee can, and some hot glue have connected to AP's over 1 mile away from the top of a hill. This is without ANY participation from the target AP at all. It's too easy actually. I'll post some instructions for this antenna, so you can see what I mean:
1. Remove the existing LNB from the dish (the thing on the end of the sick in front of the dish). Note the angle and placement of it.
2. I mount the wireless dongle in the Folger's coffee can (4 inch diameter, 5 1/3 in long). Mount it parallel with bottom can by poking a hole in the side of the can about 2.25 inches from the bottom (for 2.4 ghz wireless). Push the dongle through the hole until it reaches at least the center of the can and secure it with a bit of hot glue. This is actually a crude waveguide antenna that works quite well on its own, if you want to play around with it before mounting it. This can directs the signals that would be wasted traveling in all directions and focuses them all on the dish.
3. Place the can so that opening of the can is facing the dish in the exact same position and angle as the LNB was. The end of the can with the hole is the 'front face'. You are aiming the focal point of the dish at the hole in the can, not the dongle in the can. The front face of the can should mounted be where the front of the LNB was. Mount whatever way you like as long as it doesn't move. Keeping the center of front face of the can in the same place the center of the LNB will improve results, but the arm often prevents this (i eventually cut a chunk of my LNB arm away to achieve this, but it's not required).
4. Point the dish in the direction of the ap you want to connect to.
You must have mostly clear line of site for aps far away but the dish antenna is surprisingly forgiving, as far as obstructions that are very close to the AP like bushes. The dish will work from inside an attic even, if the roof is made of mostly wood.
Also, it might not be just you they are connecting to, even if its pointed right at you. Placing a wireless router with multiple exposed antennas in the right position (just a little closer or further away than LNB's are mounted normally) and using MULTI-LNB dish ( the older, wide, kinda egg shaped ones ) utilizes the multiple focal points and can provide similar access that will canvas a comparatively large area at shorter distances, providing access to entire neighborhoods at once in some cases.
As always the best solution is to secure your router using the latest secure protocols and a strong password.
Cheers!