Low Coolant Warning Light Question

My wife drives a 2007 RR Sport Supercharged and about a year ago I replaced the coolant reservoir due to some recall I read on here as she had the old style. Things have been good for quite some time until recently her truck has been throwing a "Low Coolant Level" warning. I've topped the fluid a couple of times to the recommended cold range, and everything is back to normal for a few months then it does it again. I'm curious to know why this thing might be drinking coolant. The truck drives perfectly and has been extremely reliable for the past 3 years we've owned it. It currently has a little over 95k on the odometer, I'd be curious to see what others think the issue might be.

Perhaps a leak someplace and I need to pressurize the system? Is this something that I can do myself?

Thanks,
Andy