I had the same thing happen on my 528xi. Opened the cover and found one of these
loose. I CLEANED the entire cover in and out and made sure there were no leaks or
blockages. Then I used an adhesive i bought at the parts store that was made for high
heat and oil/gas tolerant. Might have been black RTV? It seemed that it was originally
"welded" onto the overall cover, but for this one the "weld" had failed and so it came off.
These seemed to be screens that maybe separated liquid out of the oil or exhaust vapor
recirculation vents/airways built into the valve cover. I did that in Jan 2019 and have had
no trouble. I am not an expert. Just evaluated my options and decided it was an
acceptable risk. Also willing to take the cover off again just to confirm if I get any
indications there is a problem. I did some internet research to help confirm that the
adhesive was a reasonable choice. I used the same adhesive when I changed the Valve
that manage exhaust gas pressure inside the valve cover (the diaphram failed as they do,
and i replaced the cap without replacing the entire valve cover gasket).