Well, there is only one sure way to find out if it's the "Vacationers Home" . Just pull their meter can out, and when the backup batteries have died, a nd you now have 'Wireless Peace' in the neighborhood, you hit a winner!
Then, when the vacationing neighbor gets back from vacation, find 2 or 3 ol d microwaves at the junk yard, and bypass the closed door safety switch. Ai m those puppies at the house, and when nothing works at the neighbor's hous e, he will be calling up all kinds of folks to figure it out. After a while he will get very paranoid and after a little while longer, a 'For Sale' si gn will go up... peace will then reign in the neighborhood again. Now, if h e starts looking really tanned, and it's actually cold outside and overcast , turn down the settings on the microwaves to Low, or leave some SPF cream at his door step.
Well, it's a nice thought anyway. It gets straight to the heart of the prob lem.
Disclaimer: No, this is humor for those who are just looking in, and might want to report me to the authorities. Do not try this at home or at anyone else's home, just humor.
...and now seriously, if there is that much interference for that great an area, it is not usually because of everyday items that can be bought by a h omeowner. I only know of one customer who had an old 'Ademco' wireless syst em that was affected by his new garage opener. And that is within the realm of 'close encounters of a wireless kind'. To reach out across multiple hou ses, would be something like the 'power transformer sharing' mentioned by R TS. Also, not always thought of, high intensity heat lamps for "Greenhouses " aka 'CovertWeedFarming' can be a major cause. Those absolutely will disru pt even internet communications in a neighborhood enough to cause the "Cabl e Police" to investigate.
I don't think that I would try to find out the frequency as much as I would want to just find the 'high intensity wave source'. I do use a bulb-type m eter that indicates how much power in being put out and it is tri-axis dire ctional. Though, I don't know if I would go thru a neighborhood aiming that device at homes...I would be rounded up pretty fast by the nut police.
The serious problem with your dilemma is that even if you find out it is su ch and such neighbor, he or she will probably think that you are crazy. And , even if they believe you, they are most probably not going to change anyt hing in their house for anyone. Back to square one.
I am sure that you are using your own security brand that you like and are familiar with, but perhaps, just perhaps, trying out a different brand for this customer, using a different wireless frequency could have some benefi cial effects. Obviously, you have a serious problem in that the wireless de vices are not communicating and therefore, no reliable security.
In regards to the possibility of a cellular-phone home amplifier that can b e fitted on a roof, and wired down to a ceiling to an internal antenna, tha t kind of device would not cause interference to other homes because of its power intensity. If that was true, anyone within close range of any cellul ar tower would always have serious interference.
...mm, maybe just maybe, there is the possibility of some old-bat living in the neighborhood who was drying her hair in her sixties-era dryer that sho rt-circuited,... and she's now lit up like a Blazing Christmas Tree... the vacationer's mother-in-law?