All of the people of the house but the mom files past mom giving her kisses and then exit out the front door.

Where is the dad going? I thought he worked at home and had sliding doors in the livingroom separating his office from the rest of the house.

Alex may need to go purchase supplies at times or go to the barber shop, but those would be a fairly quick things and not involve the implied leaving for a significant absence from home. So...

Here is an attempt to offer possible reasons Dr. Alex is leaving the house in a way that would elicit a farewell kiss:

1. Alex is off to make a house call. While house calls constituted a very small percentage of patient encounters by this time, they were not obsolete. Even a few years later on The Brady Bunch we saw visiting pediatricians.

2. Alex is off to the local university/college to present a lecture or participate in a symposium, or even to teach as an adjunct Biology dept. professor for medical school students.

3. Alex is off to meet with fellow vintage auto enthusiasts who have formed a club to pursue their hobby:

https://www.imdb.com/title/tt1367545/

1. Alex is off to make a house call. While house calls constituted a very small percentage of patient encounters by this time, they were not obsolete. Even a few years later on The Brady Bunch we saw visiting pediatricians.

This is basically it, though he may be off to the hospital or some such. He's leaving because of the phone call that came in.

Later in the series he moved to an office--then it would make more sense. In one episode at his office, Donna became his office assistant--which caused the predictable sitcom arguments.

I didn't make the phone call connection.

I guess the patients would need an outside door to get into the Drs office so I thought maybe that is where he was going.