2004 Dodge Intrepid Losing coolant, but no visible leak.

Engine Cooling problem
2004 Dodge Intrepid Automatic 96,000 miles

Hi, Recently I've noticed that we've been having to put in coolant more often. Nothing really prompted it, I usually just check under the hood once a week just to see how the oil & coolant are doing. Well I noticed the coolant was under the min mark in the coolant tank thing and decided to put more in. I looked at the cap and it looked like it was in bad shape, the black washer looking part on the end was all warped and cracked so I replaced it with a new one. I topped off the coolant and that was that. Well today (about a week later) I checked on the stuff again and the tank was near empty, I refilled it to the max line and we went to run a friends house about 10 miles away to drop off some papers to him and checked the tank again before we left and it was near empty again. I've been doing some lite research and I've seen many things about a blown head gasket, but the symptoms I've seen don't really match our situation besides the loss of coolant. Here is what I've observed so far:

There is no and has never been any kind of visible leak from any where. Theres no puddles under the car or anything. The car runs perfectly normal, the temp gauge never goes beyond the half way mark like normal, there's no smoke of any kind from the tail pipe. I checked the oil and it looks normal, not gray or milky. The coolant tank looks normal. The car idles normal and runs normal. I'm no mechanic or nor am I good with cars so that's all I can observe being a novice and all. Thanks for any kind of help you can give.