Missing cover on roof ?

Opon inspection for roof seams I found I'm missing the top of one of those small round vents. I've got 3 of these on my roof but this ones larger (about 4"+) I know what the other 2 vents are for ( tank vents) but can't figure what it is for ,or what to call it ,or where to get another. Would like to post pictures but alas I'm on a iPhone and it's above my abilities .if all else fails I suppose I coul fab one up out of a pvc cap
It's just the top part