How to determine gas leak on a water heater?
Time to call a professional; either a plumber or the local fire department. They can use more sophisticated leak detectors to pinpoint the leak, and know how to do so safely. Leaking gas is not something to trifle with.

Liquid leak detectors (the type that bubble up when gasses are leaking) are great for simple threaded connections, but you've got a whole host of potential leak sources, some of which appear to be not conducive to slathering soapy water on. I think an electronic combustible gas detector is in order.