What is this open pipe?

[Hotandcold]

Hello

After moving into our house we noticed an occasional sewage smell in the cupboard/room under the stairs containing the Megaflo cylinder and various heating controls and plumbing connections. There is a toilet and shower room on the other side of the adjacent (the right in photo 1 below) wall. The property has had extensive work done over the years including 2 large extensions and the addition of en-suite bathrooms upstairs (these are fitted with Air Admittance Valves). We have our own Sewage treatment plant.

I eventually tracked the smell down to an open, grey, plastic pipe running horizontally low on the floor, just above the skirting board, and entering the wall on the bottom right of the first photo - near the corner of the shower on the other side of the wall. I sealed the open end of the pipe with the, less than professional, use of a spray can cap and some packing tape. That was 3 weeks ago and there have been no smells since and no other obvious effects. What I would like to know is the probable purpose of the pipe and whether I am risking some sort of damage by blocking it off.

The 2nd photo shows the pipework on the other side of the wall taken from the access hatch behind the WC. It is difficult for me to be sure where the mystery grey plastic pipe is routed. I believe that the vertical pipe is the main vent that runs up and out through the roof. The 3rd photo shows the shower - the mirrored wall of which may cause a little confusion
.

 

[Last Plumber]

It looks like a waste pipe that has been left connected into the soil pipe?
Can't tell properly. If it is that, you can buy a stop end (maybe a socket and stop end) to blank it off.

 

[Ric2013]

I have come across this where a WC cistern external-type overflow had been routed into the main soil stack and when the WC had been later replaced, the pipe had just been left open in the wall. Could be something similar?

 

[Hotandcold]

Thanks for these responses. Both are very plausible. Looking at the second photo more closely reveals a fiiled-in hole in the wall. It is exactly above where the open end of the pipe is on the other side of the wall. The main body of the property is quite old and there may well have been a WC with an external overflow. If not that, probably a sink. I'll fit a proper stop end.