When checking the ATF you have to have it at operating temp, not just warm. I've read 17 miuntes driving brings it to temp. Should be rock steady at the upper mark. My 4.5 will slip a little if down as little as 1 pint.
You can get a close gauge on the level cold though, but not an accurate read like you would hot. Cold the fluid will be about 10mm below the low mark if the tranny is filled properly.
Did you get any record with the car when you bought it? When was the tranny last serviced? Perhaps a fresh filter and fluid would be a good place to start. Be sure to drain the torque converter too for a complete fluid change.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.