Does Postman have an auto update mechanism built into it? Perhaps that was what you observed versus an App Catalog update. Aside from the switch to enable it and the scope there aren't any other configurable options so it's unlikely you missed anything. 

@vantive I believe what you are describing is the built-in auto update mechanism of Postman 'kicking in'.

Unfortunately the Postman auto-update mechanism can not be fully disabled which seems to be a heavily requested feature going by this thread on the Postman Github page https://github.com/postmanlabs/postman-app-support/issues/6999

Whilst App Installers will be able to install and update Postman without asking for admin credentials, the Postman mechanism is probably the one that is making the request.