SDKHook(client, SDKHook_PostThink, Radar);
SetEntPropEnt(client, Prop_Send, "m_bSpotted", 0);
SetEntPropEnt(client, Prop_Send, "m_bSpotted", 1);
Example shows all non-admins permanently and hides all admins on radar. isAdmin is my own function, just do your own admin check.
We use it for flagging campers/AFKs on everyone's radar. If a player hasn't moved X units in Y time, show them on the radar to everyone. We also use it, depending on the mode, to always show the bombcarrier on radar to promote better teamwork.