And the winner is.....
Had the chance to clean the engine compartment out and warmed the car up. got underneath the car and had it started. Instantly oil started seeping from around where the oil pressure sending unit attaches. Turns out it wasn't even finger tight.
I got lucky that it was such an easy fix. I haven't lost any oil since that fix and it has been through 6 hours of highway driving. That would have required close to 3 quarts of oil under the previous configuration.
My next question would be: how likely is it that such a part became loose spontaneously? I did have some guys under the hood updating the AC suspiciously close to the begining of the problem.
'92 190E 2.3 200K
'03 Toyota ECHO