It's almost impossible to keep secret something like the S-400 once deployed given US spy satellites. The Cuban missile crisis is an example of how hard it is to conceal large missiles, including SAM sites. (Also, specific radar emissions of SAM systems when turned on may give away their type, especially if any foreign jets are around, which is definitely the case in Turkey with a US base.)

Bombs are a bit different, much easier to conceal, but even then, see e.g. how HARM missiles were first spotted on the ground in Ukraine (from their wreckage), before there was any official ack of them being exported.

So, yeah, the answer is "it depends".

It's almost impossible to keep secret something like the S-400 once deployed given US spy satellites. The Cuban missile crisis is an example of how hard it is to conceal large missiles, including SAM sites. (Also, specific radar emissions of SAM systems when turned on may give away their type, especially if any foreign jets are around, which is definitely the case in Turkey with a US base.)

Bombs are a bit different, much easier to conceal, but even then, see e.g. how HARM missiles were first spotted on the ground in Ukraine (from their wreckage), before there was any official ack of them being exported.

So, yeah, the answer is "it depends".

As for S-400 in Turkey, Russia also appears to have been interested in publicizing the deal, which would have made Turkey's attempt to conceal/delay the discovery of the deal more difficult.