I'll throw in a couple more ideas off the top of my head.

Radio and cell phone transmitters may have generator backup. Most cell phone sites are probably battery-powered, though. Anyway, depending on the SHTF scenario, like EMP or a severe earthquake that topples the transmitters anyway, there could be unused diesel here.

How about the stand-by generators for commerical office buildings? Again, depending on the scenario, these buildings could be unoccupied and stay unoccupied, so the fuel for these generators would be unused. Like, after Katrina, I'm assuming that most of those office buildings downtown were just empty for quite a while. Imagine how long those buildings would've stayed empty if the city hadn't been pumped out so quickly.