It simply looks like a mount for some type of removable device (which currently isn't present) to me. Several councils place air quality monitors at various locations across their areas intermittently. (I've worked on traffic surveys linked to monitoring air quality. Devices for checking the air quality have been placed on mounts similar to the one you've pictured).
Ohhh I hate puzzles like this it will drive me up the wall now Zacs until I know lol. Doesn't look to have wires attached...yet ... maybe they will add more stuff at a later date?
This is probably off the wall but in Spain they have things similar to that hanging off trees and posts to catch the Processionary Moth/Worm which can be very destructive.
It looks like a siren....are you near a disused factory or large industrial site ?..we had these in our wee village years ago to signal the end of shift at the local foundry....we knew it was time to go home for dinner when it went at 5pm every day...
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