2002 Oldsmobile Silhouette system keeps draining

Unable to sustain coolent levels. Don't see any obvious signs of leak, no smell of coolent. No water in oil. Heat does not come up right away and most times it will stay cool. Eng. Sometimes spikes with heat.

I can only come up with is the following possiblities
1 intake
2 cracked head
3 cracked in system- no signs
4 heater coil - note there is a front and back heating system

Any other posiblities -?