When is a re-index for Solr started?

I'm a little bit unclear about the current situation regarding the full re-index of Solr. I had the impression, that a full re-index is only initiated when administrator calls PUT /api/item-reindex or when backend-container on intial start finds out that the Solr is empty. (and there should be also a re-index in the night on base of a cronjob but that seems not to touch the already existing Solr index)

The last days have led to question my assumption on reindex of Solr at intial start of the backend-container. Could it be, that every time the backend-docker-container is deployed an automatic re-index of Solr is initiated? Thus (sometimes partly?) deleting already existing solr index in the persistent solr-part of the backend-docker-bundle and making search endpoints to deliver no (or only partly) data?

@ekalata Is is possible to have a look into the code and to sum up the situations when a sorl reindex is initiated and under what circumstances this happens?