Leaky overflow reservoir

[taosx5]

Hey out there. Hoping someone can point me in the right direction. I have had my new to me 03 X5 4.6 since July 09 (currently 68K miles). On my first x-country trip, I noticed a small amont of anit freeze leakage out of the radiator fill and overflow reservoir. The temp never got above the mid range and half way thru our 4K mile trip I washed it off and it did not reappear for over a month. Last week I noticed it again and examined but could not find a leak, so again I washed it down and am trying to keep on eye on it. I have had to add about two cups of antifreeze in 3 months and 7K miles. It appears to me that the reservoir is a molded piece of plastic and there is a seam that separates the top 1/4 from the bottom 3/4. My guess is that it might be a small leak at this seam. It does not leak enough to leave any fluid while parked in garage but seems to more like steam leakage under pressure. I suspect what is happening is that when the temperature gets up to operating temp, pressure in the system builds up and it finds the weakest point and leaks off as steam. Any ideas anyone? Thanks in advance for any advice.