Help Identify Please

I found this object in a couple of spots in a house.
I believe it has to with the whole house ventilation system.
Object is round about the size of a smoke detector, but twice as thick.
Top pulls open about 1 inch but springs closed.
Inside looks like it is a gray filter screen.
There is a string hanging down from the object.
The object is mounted on exterior wall and has louvers on outside wall.

It is a Passive Whole House Ventilation system. Denmark has them everywhere. Here in Canada we use HRV’s because the house needs to exchange cold air to be beneficial.

Thank you very much. The mystery has been solved.