The authors report on measurements of the magnetization of metal sinters made of different Ag powders with av. (nominal) grain sizes of 40 nm and 70 nm at 1.6 K £ T £ 250 K and 0 £ B £ 6T. Besides the dominating diamagnetism of Ag, the sinters show unexpected magnetic properties; e.g. once exposed to a magnetic field B, the sinters keep a permanent magnetic moment (in zero external field) which strongly depends on the magnitude of B. The possible impact of the unexpected magnetic properties of the Ag sinters on various exptl. observations at very low temps., e.g. the magnetic field dependence of the thermal boundary resistance between an Ag sinter and 3He at millikelvin temps. is discussed.