Will more than likely be oil from the lowers – it does go quite a long way. I had a similar thing the other week with the air side of my Reba’s while camping. It could only be the lower leg lube and made quite a bit puddle.
What Reba’s are they? I’ve got the dual air with remote lockout (with the small gold floodgate knob) and have had oil leak out when the fork is upturned, where the damper assembly passes through the top of the fork leg.
Also, it’s quite possible that a small quantity of damper oil has leaked into the lowers over time (this seems to happen with most RS forks), but from the sounds of it the puddle you mention sounds about right.
You could up-turn the fork, remove the nuts from the lowers, lift the lowers off the stanchions by about 30mm or so & then squirt a bit more lube into the damping side lowers using a syringe. Problem is that by the time you get it all bolted back together, the new oil will probably be starting to leak out.
You probably need a new o-ring at the top of the damper leg to fully cure this, although a strip & re-build might help for a short time.
There should be a full technical service manual on the Rockshox/SRAM site for your fork. It’s quite straight forward to service them, so long as you have new o-rings, circlips, oil & the right tools.