The Geotrigger Service relies on the GPS within mobile devices, and you're correct that they have a hard time knowing where they are indoors.
While Esri doesn't provide a solution or the infrastructure for indoor geolocation, the Geotrigger Service supports the ability to plug in your own coordinates and not use the local GPS. In order for this to work, an existing infrastructure needs to exist, such as iBeacons or an indoor geolocation system. Esri does not provide this, but if one already exists, the Geotrigger Service can leverage these points and work as intended. GIS Inc. is an Esri partner that has used indoor location in conjunction with the Geotrigger Service, but we don't know if they've chosen to productize this proof of concept.
I hope that helps! The docs have more information, and you can also ask questions in our Geotrigger SDKs and API space here on GeoNet.
