corsa sxi?

hi,i have a corsa of 5 years old and its begining to cause a problem or two,can anyone tell me why the radiator water bottle goes empty after a weeks running of roughly 100 miles when there appears to be no leaks,and also there is a thick scummy layer on the oil cap when i go to top it up which is also disappearing quite fast,the engine management light also comes on every now and again when its started from cold.
I would be grateful for any help recieved.

Thanks.
Jock.