Could also have been a connection to a pan fan – a type of extractor fan that is connected to the flush pipe and sucks the smells from under the pan ring when the flush pipe is not being used to flush water.
This is now becomming a more popular venting system to eliminate the “smells” associated with enclosed toilet systems.
Hope this helps.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians