I have been experiencing random ground units some times beeing invisible in multiplayer, up till the time they are destroyed by anyone else, at this point i can see de destroyed veichle, I had this with SA-10 launchers (not the whole group), ZU-23s emplacements and manpads.
Also on ocasion sometimes other player's aircraft do not appear until they rejoined the server, in spite of beeing flying and active on the server