My coolant is empty all the time!?

Hi i noticed one day my coolant was empty so i filled it up a few days later my engine overheated and the engine management light went on and was because my coolant was empty again after a few day! i fill it up 6 times until it had leveled out! i live in dartford and drive to crawley every day and which is an hour and when i have completed my journey it is 50-75% empty! There is no visible leak anywhere! Not in the foot wells or under the car i took to to a garage an they couldn't tell me what was wrong but charged me a ridiculous amount to find out! And now i have notices there is a white residue on the cap. Someone help me please. Has this happened to you?