Did you ever see a 1-1/2 inch undersink bathroom drain cleanout with a plastic cap glued inside completely blocking it from the inside?
Never saw anything like that, very strange. Put a hole in it with a drill and then try to hook it and pull it out I guess.
My bet is the base of the sink floor is now higher than an old one and the installer could not figure out how to connect the trap with it being so low. Since it is apart, maybe shorten the tailpipe so that first union is just higher than the outlet. Save on a drip drip where the standing water is currently above the tailpiece connection to the trap.
That's what it looks like to me. At one time that was the drain, now the drain is higher and that had to be blocked.
I thought the question was why there was a strange plastic plug inside the cleanout, behind the normal screw in plug. If the question is why is the cleanout located where it is, that's not unusual, they put it wherever it's convenient and a good place to let a snake enter easily.
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