This later confirmed why the toilet was leaking and needed to be reset in the first place -- they had 3 kids so I'm pretty sure that had a lot to do with it.  

Other than that, a refrigerant leak, the fan motor burns out, just throw it out and get a new one.

Or I could dig up an irrigation leak, zero knowledge required.

One, expected, but arose sooner then expected, was re-piping the house due to the old galvanized pipes causing low water pressure & continuous leaks that were needing to be repaired. 

Gas, fast food, etc.   

It has been recently introduced and is supposed to put a cap on the cost to install utility hook ups for new construction and ban hedge funds and corporate entities from buying single family homes in California):SB 1210"This bill would, for new housing construction, prohibit a connection, capacity, or other point of connection charge from a public utility, as defined, or a special district, as defined, for electrical, gas, sewer, or water service from exceeding 1% of the reported building permit value of that housing unit.

I have a few questions about handling this situation:- should I hire and pay for a plumber to go into their unit to cut open the ceiling and identify the source of the leak? 

I strongly recommend not getting a refrigerator with a water hookup they often leak and damage the floor or the downstairs unit or attract mice to the source of water.

I could move the water and gas line (for the water heater) to the other side of the wall which is the garage.

The gas bill has been coming in pretty high lately.