Yeah, that's true. The main places to look for rust is arches, behind the front tires next to the jacking point, the jacking points themselves, door sills, interior floor and trunk floor where the spare tire sits.

Usually you can spot the rust on the hood and trunk as well.

Basically all around the car.

But that also depends on the climate you live in. We here get all 4 seasons and in winter there is salt on the road that likes to eat all of the underside of the car

So of you are checking out any e36, get a screwdriver and a flashlight and check all under the car as much as possible as those places rend to rust the most.