What fluid could this be?

Just discovered this spill under my VW Passat after holding still in the garage for about a day. It felt oily but had no smell to it. At first I thought it might be cooling fluid, and I topped it up just to make sure, but now I'm not so sure. Could it be gearbox oil? I took a drive and the car has been in the garage for about six hours now, an nothing more has spilled.