Thanks for the pointer Ismail,
We have a lot of Factories to create viewModels, I'd like to re-use these to generate indexable content... however they all rely on Umbraco.Context, which is not present when indexing. The most common issue is any call to GetPropertyValue<string>(...) will fail when it hits Umbraco.Web.Templates.TemplateUtilities.ParseInternalLinks But I'm guessing macros will fail also.

Is there a way around this?
Create a fake UmbracoContext? currently, my factory has ugly if statements like this:

ExpandCollapse viewModel;
if (UmbracoContext.Current == null)
{
    viewModel = new ExpandCollapse
    {
        Heading = nestedContent.GetProperty("heading")?.DataValue.ToString(),
        // snip //
    };
}
else
{
    viewModel = nestedContent.Map<T>(); // using https://github.com/AndyButland/UmbracoMapper
}

Simplified Example:

protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
{
  var helper = new UmbracoHelper(UmbracoContext.Current);
  ExamineManager.Instance.IndexProviderCollection["ExternalIndexer"].GatheringNodeData 
        += (sender, e) =>   Indexer_GatheringNodeData(sender, e, helper);
}

void ExamineEvents_GatheringNodeData(object sender, IndexingNodeDataEventArgs e, UmbracoHelper helper)
{
    //check if this is 'Content' (as opposed to media, etc...)
    if (e.IndexType == IndexTypes.Content)
    {
        var page = helper.TypedContent(e.NodeId);
        var title = page.GetPropertyValue<string>("title");
        // NullReferenceException in "Umbraco.Web.Templates.TemplateUtilities.ParseInternalLinks"

is working on a reply...