2D X-Ray Imaging of Surface Mounted Capacitors
A microfocus xray image (the standard thing for imaging PCBs to check soldering) can see down to a scale to make the bond wires at least somewhat visible, so any feature above that size should be easily visible. 

A cap is not the obvious component if you were seeking to hide an antenna structure, and inductor is a much more obvious contender as the things tend to radiate anyway.

Given a sufficiently high operating frequency it might be possible to make a on chip aerial structure either in the metalisation or possibly even in the doped silicon and especially in the Si case, good luck seeing that. Efficiency would likely suck however, and the frequency would have to be high enough that penetration would be poor. This is the sort of thing you see in THz imagers and the like, expensive tech and possibly not that compatible with standard cell libraries.