After playing a lot more, I have actually found "loose" .45 ACP ammo on top of police cars in Sveltojarsk _twice_ (in varying amounts - just under 10 and 40).
However, there's no certain way to know if they were dropped by players or spawned by the server. All I can say is that cars' trunks and police station's doors were closed.

I've experienced this as well across variety of servers, both regular and hardcore mode. Neither ammo boxes nor individual bullets appear in seemingly any location (including military bases). However, 1911 and FNX magazines do spawn with bullets in them.