:homer::homer: If those bolts was undone your compression was lost for ever i would say..thats why we all keep them tight and check them every now and then..LOL You are joking...right?
If those screws are not tight you will have oil all over the place cause seal gasket isnt sealing properly..
happens on all that I've seen mate. they all rattle loose from time to time.
And no, it will not reduce the compression as its just a cover for the cams, keeps the oil in. Worst thing that can happen is you lose some oil as it lifts slightly.