The best times to inform humans of your dish's emptiness are when they are unable to ignore you, such as when they are sleeping or on the toilet. Should you catch something of your own outside, it is only polite to attempt to get to know it. Be insistent - your food will usually not be so polite and try to leave. They are very protective of what they think is their property and will object strongly if they catch you sharpening your claws on it. Dripping taps are the best sources of fresh water.
Toilets are the next best.
It is imperative that any sound of running water be investigated immediately for a possible drink. A plaintive meow and licking the faucet usually will get most humans to turn on the tap. Humans will turn into raging monsters while under its influence, sucking up all the carefully shed cat hair and terrorizing the feline residents with evil glee. All you can do is run and hide. Occasionally, the humans are forced to open the vacuum cleaner and remove a swollen bag from within.