We’re always a little surprised by how well a vacuum thermos works, but eventually the contents will cool down (or warm up depending on what’s in there). [Gamesh_] added a temperature meter to his thermos using an Arduino and a temperature sensor. The original post is in Portuguese but [Bruno] republished it in English.
The temperature sensor has been repurposed from a digital thermometer meant for taking your temperature. Holes for the LEDs making up the indicator bar were melted in the side of the plastic housing. When the hot liquid is poured out at about 0:45 into the video you can glimpse the Arduino hanging our on the other side of the pot and a power cord running off behind the laptop. It would be nice to see this migrated over to a less powerful chip and run from a small coin cell, but we like the concept.