Take your oil cap off and have a look underneath - if there is a white sludge then there is water mixing with your oil (usually through the head gasket) and as Billb says if you're losing water into the cylinders she will be belting out white smoke (steam) from the exhaust, if not you're OK. I have just helped a friend with a similar problem - He was losing water by the gallon but there was never any evidence of a leak. I popped the bonnet took a look at the pipe which goes into the water pump (this was an e34 mind) and realised it had been replaced, now, the part of the water pump which the hose attatches to is plastic (other manufacturers use metal) and I figured when the hose had been replaced someone had overtightened the jubilee clip and put a slight crack in this plastic part. I asked him to start the car and get her to operating temp. Sure enough just around the end of the hose to waterpunp joint was a little wisp of steam. 3 days later he informed me that I was right (he took it to a garage and they said exactly the same), that he'd replaced the part and there was now no longer a problem.
I'd spend a little time hunting under the bonnet for steam from all the hoses and joints - you need to get her to operating temp though as when cold there is not enough pressure in the system for the leak to occur - hence no puddles - heat her up nad the pressure will soon start to force steam and bubbles from the tiniest of cracks (some which are barely visible to the human eye).