Sometime recently, I knocked the cap off the grey tank vent with a tree branch.
In preparing to replace it with a new one, here's what I found under layers of caulk, silicone, and maybe Dicor.
The outer ring is the deteriorated remains of a solar vent. When it sun rotted and presumably fell off, they left it there. Inside that is where the vent base I removed was.
After a lot of scraping, here's the cleaned up area before I wiped it with solvent.
Note all of the screw holes. There are at least 3 sets, from 3 different vents. Per previous work done by the P.O., no butyl tape was used to seal any of them and their screws, and some of the galvanized screw shanks were rusty from wicking water.
Not shown (I guess I forgot to take a picture of it) is the only semi-good thing I've found that the P.O. did. There is a custom plastic gasket that looks like it was cut from the cover of a 3 ring binder that fits over the pipe and seals the annular space between the pipe and roof. I couldn't think of a reason not to reuse it, so I did.
Here's the finished job. The gasket was glued down with Lexel, then the vent base stuck down with butyl tape. I wrapped a little of the tape around the stainless sheet metal screws before I drove them in. Then, everything got a topping of self leveling Dicor, including any remaining exposed screw holes.
Based on observation and experience, someday soon I need to tackle that adjoining shower skylight and see what wonders lurk beneath it. There are 3 or 4 tubes worth of gray silicone rubber sealing it down.