Recently I went into my garage to find the invisible dog fence transmitter beeping. After calling the company I was informed that this alarm meant there was a break in the fence, and that I should have them come out as soon as possible - because my dog would surely figure this out in a few days. And hey, for only $100 an hour, they would be happy to help! Long story short, they tried for an hour then told me the yard was too overgrown (half the yard is wooded) and they would have to come back in the fall when everything died back (2 months away). This made me swear that I would do it myself - but after digging up over 100 feet of wire and driving myself crazy for a few days, I had not found the break. Then I found this strategy on the internet (link)
that cost me under $20 and worked like a charm. Hopefully this makes it clear for everyone else and saves you some money and frustration. Good luck!