Losing coolant

My current truck (the one I will soon be getting rid of) is starting to lose coolant over a rather short period of time. For the past few years I've had to add coolant about 1 or maybe 2 times a year, never really thought anything of it as it wasn't that often that I would have to add some. Well, I just added coolant back in mid-January when my "low coolant" light came on. And just yesterday my "low coolant" light came on again! I just added coolant 2 months ago and now I had to again?!

I can't see any leaks on my garage floor and it doesn't appear to be leaking while the truck is parked. I have a feeling that this will be a fairly expensive repair.

Anyone have any idea if this actually will be an expensive repair or what might be the problem?