μSR spectroscopy down to 40mK was carried out on PrAu 2 Si 2 . This compound and heavy fermion URh 2 Ge 2 are the only stoichiometric spin glasses reported among the tetragonal FT 2 X 2 (F=rare earth or actinide, T=transition metal, X=metalloid) materials. Although bulk measurements on PrAu 2 Si 2 exhibit all the typical features of a canonical spin glass with a freezing temperature of ~3 K, no evidence for the formation of a frozen spin-glass state was found with μSR. Instead, the data clearly demonstrate that the magnetic moments in PrAu 2 Si 2 remain dynamic down to the lowest temperatures. The discrepancy between these observed dynamics and the spin-glass-like response in bulk measurements is not understood. Analogous measurements on PrAu 2 Ge 2 and PrAu 2 (Ge 0 . 8 Si 0 . 2 ) 2 showed the expected antiferromagnetic signals, demonstrating that in this type of alloy muons detect magnetic properties in the usual manner.