Antifreeze usage

We are using a lot of antifreeze. There are no leaks detected. Rad is only a year old, no problems there. I've checked everywhere for any sign of a leak but there is nothing. Mechanic says this is normal for "winter". It's not that cold here in Vancouver, B.C. Canada, so I don't trust what he is saying. Every week I have to add more antifreeze. Please help as this is getting expensive!