As soon as you have a neutral/ground connection anywhere and bonded water pipes the pipes can and will become a return path. The whole stray current issue is the main reason why some distribution network operators in Europe prefer TT systems where the building grounding conductors are only connected to rods/UFER but not to the neutral. The neutral is only earthed at the transformer. The downside of TT is obviously the considerably higher impedance, requiring additional earth leakage protection (i.e. an RCD for all circuits).
Re: impedance of rods: that totally depends on local conditions but 5 Ohms is pretty low. Very low for rods in fact. When ours was newly installed it measured at slightly over 8 Ohms and I'd say we've got decent conditions here (mostly clay). By OVE standards, up to 100 Ohms are considered acceptable.
