I would say, follow up with tracing out those circuits and find out where they actually run and exactly what is connected to each of them.
I have seen mysterious problems similar to this in the past come to a logical conclusion.
In one case, it was a building with a commercial retail shop on the first floor and a small residential apartment on the second floor. The long-term apartment tenant complained to the owner that for some reason, her electric bill this season was suddenly consistently high at close to $300.00 a month during the summer when she was hardly ever even home and was not using any air conditioning.
It didn't take very long to find the problem. Turns out that someone else from the same company I worked for at the time, had installed a new circuit for a dehumidifier in the basement of the commercial area about 6-months before and had mistakenly tied it into the tenant panel, which was located in the basement right next to the panel for the retail space. [Existing installation]
The funny thing is that the electrician that actually did the install was sent back to troubleshoot the problem a couple of weeks before and after some in-depth diagnostics, his answer was that the electricity "must be bleeding to ground somewhere".
Not to say this doesn't occur, but it seems like for many years, I've heard this excuse used all too often when someone can't locate the actual problem.
