He didn't say it was a decent sized puddle. Just that it was leaking. You need to start covering all your bases, Bill. You know better than that.
But to the OP, you mention that the tranny fluid is going getting low. If you have a leak, and you notice that one of your fluids is getting lower, I would put my money there first.
As for not changing the fluid on the tranny, I don't agree with that. If it were me, I would drop the pan and change the filter and replace the fluid that you loose. It will only change out about 50% of the fluid in the tranny and allow the old to mix nicely with the new. The issue comes with an old, beaten, hurt tranny that someone tries to replace all the fluid at once. It is a shock to the transmission and usually not recommended. I would venture that is what your mechanic is talking about.