Residential wiring troubleshooting Q

I have one of those RF "breaker finder" gadgets that I've used with good
success. It's a basic no-frills type that
doesn't trace open circuits or anything. I was doing some electrical work at
a friends' house where I wanted to turn off a breaker to a branch circuit
I'd be working on. When I did my usual test with the breaker finder, it
would indicate just about all the breakers in the panel were the right ones.
I know this is impossible, but I'm just curious why this might happen. I
adjusted the sensitivity of the receiver as appropriate with the same
results. The receptacles on that circuit were working properly and tested at
roughly 120 volts. (if that helps) Any ideas are welcomed.