That could be.
They may have run the washer into the lav vent.
Once the walls are in, it's impossible to determine what is there.
Just sort of a check for understanding.
When we were first looking at this 1960's tri-level, one of the first things I noticed was the washer/dryer on the top floor. Interesting. Well, I figured, it's convenient and there is the slight possibility it was done right.
So this week I noticed for the first time when the washer is draining, a little soapy water comes into the bathroom sink right below it, one floor down. Goes back out, too.
Without cutting into any walls, methinks: previous owners installed washer/dryer upstairs, but probably (a) undersized the drain and (b) likely didn't vent the thing at all.
they probably installed the laundry into the
lavatory line downstairs...below the drain line and
are teh vent does double duty....
most likely it is beginning to stop up and it might
be wise to have that line cleaned....
some day you will probably have something similar to
"old faithful " spurting up in your sink every time
you use the laundry and eventually its possible to have a
substantial flood downstairs....
get the line cleaned...