91 buick skylark-transmission low fluid??

Hello new to this...I have a 91 buick skylark 2.3L WITH 190000 miles on it--looks like poop but very realible---noticed trans slip after stops--check fluid was very low (so much for tech checking ALL fluids with oil changes)--my question is WHY---no trans fluid spots on driveway (lots of oil spots from oil pan gasket) so where did the fluid go????