So far I always took a multi entry permit along with my "retirement visa".

What's the situation like when you don't have such a permit, when to get it?

And I remember vaguely that the retirement visa could become void if you didn't arrange such a permit properly?

To exit Thailand and protect your permission of stay you need to obtain a reentry permit.

This can be done at your local immigration office or airport at day of departure 

Single reentry 1000b

Multiple reentry 3800b 

 

At airport they provide option of 200b to fill in form provide photo etc.

Here is thread.

 

You need to have a re-entry permit if you are on a long stay visa or extension. If you dont have it and leave the country your visa is over. You can get a multiple entry anytime at an immigration office for 3,800 baht. Or you may buy single re-entry permits for 1000 Baht. These can be obtained at an immigration office, or if left late at both Don Muang and Suwannabumi Airports. There is a charge for photos and filling in the forms at the airports. 

 

Leave with no re-entry permit your visa is finished.