According to the Air Force Chronology for 26 August 1944:
MEDITERRANEAN THEATER OF OPERATIONS (MTO)
STRATEGIC OPERATIONS (Fifteenth Air Force): 470+ bombers escorted by P-38s
and P-51s attack targets in Italy and Rumania; B-17s hit viaducts and bridges
at Venzone, Avisio, and Latisana, Italy;
in Rumania, B-24s hit a train ferry
and terminal at Giurgiu, Otopeni Airfield, barracks and troops in the Baneasa
area and a viaduct at Borovnica. (I believe the Borovnica Viaduct was a Railroad Bridge/Viaduct in Slovenia.)
(Otopeni and Baneasa are just north of Bucharest, Romania.)
According to this link:
https://groups.google.com/forum/#!to...ia/LRaPmwuwWio
the P-38 was downed near Vratsa, Bulgaria.
There does not seem to be a MACR for any P-38s or (F-5s) lost on 26 August, 1944 (but as has been noted there are missing MACRs). P-38 groups in the 15th AF were the 1st, 14th and 82nd FGs. A list of August MACRs can be seen here for example:
http://www.aviationarchaeology.com/s.../44AugMACR.htm