It doesn’t always indicate anything is secret, they just don’t want anyone flying near it for some reason… Those markings are such small areas you could easily see into that area without flying in the restriction.
In the U.S. restrictions like that are common around tall antenna farms, or tethered radar or camera balloons. If you look along the mexico border with California and Arizona you will see some of these where they have surveillance balloons tethered to 15000ft. Those tethers are hard to see, so they just don’t want you flying there as it is a danger.