On the Metro network, it is free exit. You feed your ticket or tap your Navigo at the start of your trip, and to exit it's as simple as walking out.
On the RER and Transilien, at urban stations, you feed your ticket or tap your Navigo at the start, and at the end, you have to validate to exit the system.
The validation on exit is to ensure that you do not exceed the limits of your zones, for example, if you attempt to take RER A from Châtelet–Les Halles to La Défense on a T+ ticket, you will be stopped by the faregates at La Défense, since you are not allowed to use T+ tickets to get here. You also risk being caught by fare inspectors and getting a fine.
But some stations don't have turnstiles, they instead feature simple validators at the entrances where you are expected to validate your ticket or Navigo.
- At older stations the validators look like this. The ticket feeder doesn't even encode the magstripe; it just stamps the date, time, and station code on the back:
- Newer stations are more likely to feature these new validators. They are able to validate the same way as an entrance turnstile, encoding the magstripe and printing data onto the ticket:
At stations without exit turnstiles, is it expected that we "tap out" or validate our ticket a second time as we leave the station? Or are we allowed to simply "walk out" and go to our destination without problems?
Thanks for taking the time to write a precise & detailed question.
According to the Terms & Conditions for Navigo passes, you are required to tap both when entering a fare area/vehicle and when making transfers or exiting the fare area/vehicle (when applicable).
Here's the particular point related to this, available only in French:
6.1 Le Titulaire doit obligatoirement et systématiquement valider le support contenant son forfait Navigo Mois ou Navigo Semaine sur les appareils de validation des transporteurs avant chaque trajet lors de son entrée sur le réseau et/ou de sa montée dans le véhicule, mais aussi, le cas échéant, lors des correspondances et en sortie, sous peine de se trouver en infraction.
So if you are leaving a destination station and there is a validation station (and no gates), you're "supposed" to use it for your Navigo pass. Have I seen people depart the station without doing so: most definitely.
The last time I looked at the fines related to failing to validate a valid weekly/montly Navigo pass, it was 5€ fine. (i.e. Failing to validate when entering a fare zone / vehicle to begin travel.). I imagine the principal reason validation is requested for unlimited use pass holders is for accounting & business intelligence purposes. Understanding ridership is likely a key metric for planning/staffing/maintenance, etc.
With regards to single-use paper tickets, when there is no exit turnstile/gate, you would just walk out of the station. You wouldn't use the stamp machine (composteur) again. Those are only for ensuring the ticket is valid for travel for the next XX minutes and cannot be used again.