The previous founder wrote in cache log that he had dropped a trackable called GeoKret. I've found it, but I don't know how to log it? I can't find it on GC site.
GeoKret is an open alternative to commercial TravelBugs. It is an independent solution, having own site: geokrety.org, where you can log it.
It is integrated with OpenCaching portal, but Groundspeak is not willing to integrate them with their site. You can use Chrome plugin called GeoKrety Toolbox.
Registering new GeoKret costs you nothing, but you need to print a label yourself and laminate it. There are a few templates for labels available, my favourite has QR code which directs the founder directly to the site where he can log the GeoKret.