Answering orig post..
Yes, have them call an electrician.
IMO an electrician might..
1.Verify that IS the right breaker you are looking at there.
2.Test for voltage (any voltage) at the sign.
and if 0v....
3.Continuity test the buried conductors.
You need a permit and an electrician of course (and the original electrical plans will help), but pulling a feeder from another sign or light is not necessarily a no-no. The amp load, conductor size, run length/voltage drop, overcurrent protection, etc all must be considered though.