I am not saying that there aren't invisable planes
as there are but :
I',m sure a few are down to the 'ALT i' icon command.
example:
I like to check what people/squadmates are flying using the 'ALT I' command and on a couple of occations I have stupidly forgotten to put icons back to 'Normal' I might have only friendly icons displayed for instance, result: some guy below you is very hard to see as hi doesn't have the big red icon over him pops up behind you with his tracers disabled and BAM your dead and you think you've been got by invisable plane

I come across a few people who dont now this command, so remember check message window says 'normal icons' if you use this command

just thought I'd say as I haven't seen this raised before, thanks