Occasional brown marks on clothes after washing

Does look like rust to me too.The stain pattern does not follow the drum hole profiles = so we can at least discount a problem with slushy film under the drum, discount grease from failing bearing/s (in which case the drum bearings would be shot which would mean the drum would be noisy – and as you say this is not the case, no iffy noises at all). I think we can also discount the gap between the door seal and the front of the drum – where sludge can build up. The bearing spider is an aluminium alloy and they do not corrode like that deposit / stain.

Have you checked in the flange of the door seal at 6 o/clock position for any metal rusting objects? e.g. paper clips, key rings.

The only other thing I can think off is a rusty escapee bra wire underneath the drum that lies there but not causing any noise issues which they would normally be expected to do – not all bra wires are s/steel so they can rust but even so that is not common to cause staining but still remains a possibility.
Check within the door seal first as dismantling the tank is not for the faint hearted especially if you have never done it before – that would be your last resort and in most cases you would not do that as you move the heater to find and remove caught objects. It is equally possible that the problem has got nothing with the machine. Tumble dryer problem perhaps: if so and if a Zanussi/electrolux when old the front drum support felt produces brown marks on clothes though not necessarily as spots. Other brands are not immune to that too.