2000 toyota camry coolant leak

My wife's 2000 camry will NOT stop leaking coolant. We have taken it to the mechanic twice to replace some old radiator hoses, but they could not find any evidence of a leak. They "hooked it up to pressure" and even kept it over night and said they didn't notice anything. However, after having it for a week, when i went to check under the hood today, the resevoir was completely empty. Any ideas?