[RelSpecificity]

If it is the diff, there probably won't be a sensor for the oil so there won't be a warning and there won't be a limp home mode activated. Although, if there is a sensor in there, it might.

Unlike the internals of an engine, the transmission/diff isn't quite as critical to be properly lubricated. It's not going to seize up and destroy itself like an engine that's starved of oil. The transmission & diff (which share the same oil on a FWD) are basically just big boxes with some gears in them. They certainly need lubrication, and long term neglect with very poor, or no lubrication can damage them, but I doubt you would have lost enough to do any damage, and you could probably keep driving as normal for weeks at a time with no serious damage.

If you can still change gear easily and smoothly, then chances are you've got stacks of oil left, so don't panic.

Having said that, don't ignore the issue and get it to a mechanic (which you're doing on Friday).

The only other fluids in the engine bay is brake fluid, power steering fluid, coolant and engine oil; but generally those aren't located where they can leak onto the exhaust (with the exception of the engine oil). So it's either a leaking engine oil from a gasket, but if your engine oil levels are ok, then it's most likely a drive shaft seal on the diff.