I have located few of them on my last bikepacking trip. Some of them are already mapped in Estonia. Would be great to see a map for it or some tool to see them since wiki tools did not give me any help on it.

Thanks! I now know what they were! There are still lots of them surviving in Estonia and even some of them are on the national hiking trail to present as a historical part albeit without any sign or post to explain what they are. Just structures, a relic from good ol’ times.