I suppose there's an outside chance that they did it entirely in PCB, and the SCH doesn't exist at all (and perhaps they're embarrassed to that end?).
Like, I did some updates to a board recently, in the same vein (it was imported from other software, and the schematics didn't transfer). Even just doing netlist updates is such a PITA in Altium, I'd hate to have to do a whole board from scratch... but who knows. There are almost always at least three ways of doing something in Altium, and people can develop (and stick with) weird workflows...
If nothing else, you can extract the netlist. It's a rather dry (hard to read) format, alas, and not very rich either (i.e. no pin names (depending on flavor of netlist), descriptive symbols, sense of signal flow, etc.).
Assuming the PCB uses nets at all... shudder?
Tim