Last night, SAN was totally fogged in and the airport was closed. All inbound flights were diverted to airports like LAX, PHX, LAS, and OAK. It is very possible that the DL 757 you saw was on its way to SAN but could not land there. I know for sure that a CO Md-80 was forced to divert to OAK, if you saw one today that shouldn't have been there.
P.S.- Since most if not all of the aircraft that were forced to divert were scheduled to be used on morning flights out of SAN, all these planes flew from the diversion airports to SAN this morning, and had performed their turnarounds from about 10:00AM to 12:30 PM. I went down to the airport to take some photos of the rare occurrence, and it was pure havoc. Planes were waiting as much as a half hour for gates, and entire sections of the airport had to be closed to nonticketed passengers. I got some great photos though!!