We analyze the electron spin relaxation rate 1/T1 of individual ion-implanted 31P donors in a large set of metal-oxide-semiconductor (MOS) silicon nanoscale devices, with the aim of identifying spin relaxation mechanisms peculiar to the environment of the spins. The measurements are conducted at low temperatures (T100 mK) as a function of external magnetic field B0 and donor electrochemical potential μD. We observe a magnetic field dependence of the form 1/T1B50 for B03 T, corresponding to the phonon-induced relaxation typical of donors in the bulk. However, the relaxation rate varies by up to two orders of magnitude between different devices. We attribute these differences to variations in lattice strain at the location of the donor. For B03T, the relaxation rate changes to 1/T1B0 for two devices. This is consistent with relaxation induced by evanescent-wave Johnson noise created by the metal structures fabricated above the donors. At such low fields, where T1>1s, we also observe and quantify the spurious increase of 1/T1 when the electrochemical potential of the spin excited state |↑⏐ comes in proximity to empty states in the charge reservoir, leading to spin-dependent tunneling that resets the spin to |⏐↓. These results provide precious insights into the microscopic phenomena that affect spin relaxation in MOS nanoscale devices, and provide strategies for engineering spin qubits with improved spin lifetimes.