Some jackass installed a grounded receptacle so they would have the convenience of being able to plug in things which require grounds. However they did not actually hook up the grounds. That's why they ring out to nothin', as SimonB says.
The small voltages you are seeing are because you are using a DVM (most cheapie DVMs are very high impedance, which is generally a desirable feature, but is sensitive to phantom voltage). If you used a low-impedance meter that voltage would disappear.
Anytime you're dealing with old wiring like that, your first priority is to fit RCD (GFCI) protection first on anything near water (dockside, pool, bathroom, kitchen), then on lesser damp locations (basement, garage, outdoor outlets), then on the rest of the house. For human protection you want 5-6 milliamp trip threshold. That is too sensitive for a whole house, so it needs to be applied circuit by circuit. Euro-style "whole house RCD" is necessarily a weaker 30 milliamp trip threshold. It is better than nothing but I would want supplemental protection on the above priority list.
What's the difference, 6-30 ma? That is the "Stun zone" where it's unlikely to kill but likely to stun. When you stun, you fall, and if your face lands in water, that's a kill. It's called "electrical drowning". Happens all the time.
Your second priority, to do after or (if unable) instead of RCD/GFCI, is proper grounding. Most countries around the world crib their electrical codes from each other. In 2014 the US NEC liberalized the rules, to allow retrofitting a ground wire on any circuit. That means the ground wire can be simply run alone, and can even go to an intermediate point that has a thick enough ground wire. That may have made it to your country, in which case, go crazy :)
I am of the view that ground fault/residual current protection is pointless for an installed, non-movable machine with a metal chassis that protects all the electricals; and which also has a competent ground back to a panel with a proper neutral-ground equipotential bond. Any ground faults will reach the metal chassis/shield before they could possibly reach a human, and be directly routed via the ground and N-G equi bond back to neutral. A minor leak might just waste power; a bolted fault will return a massive amount of current through the competent ground, causing an immediate breaker trip.
The purpose of the N-G bond is at least twofold:
- to return fault current to neutral (remembering electricity wants to return to source not ground, actually the N-G bond is what makes ground a desirable return); and
- to prevent the otherwise-isolated electrical system from floating at some insane voltage due to a transformer leak.