temperature values

Isn't it the actual "ambient" temperature measured by the watch? It would be in fact ambient temperature raised by the body heat (so basically useless value). 

A Vivosmart has no temperature sensor, as far as I know.

Hm, in such case I have no idea from where does it take the temperature readings. But generally speaking, Garmin connect app shows watch temperature sensor values in the activity graph and it also corresponds with what I would expect it to be (start lower and increase temperature with activity). 

It suggests the VF5 does have a temperature sensor, but it is a useless metric. The other day I did a walk in -3C  weather, got 23C at the start (leaving the house) and it dropped to 15C in an hour. Time to get some new gloves I guess...

No, the Vivosmart 5 does not.

Where do the temperature readings come from then?

Because the watch is tethered to the phone, and the Connect app gets it from the weather connection/app/widget on your phone.

Again, the VS5 does not have a temperature sensor. I'm not even sure you could connect an external tempe sensor to the watch. Some of the more expensive models can accept a tempe sensor. and even have their own internal sensor.

It's freezing outside, my temperature graph for indoor activity starts at about 25C and gradually rises up to 28C during 1 hour excercise. Not sure what place on Earth does it take the weather report from, but it definitely isn't from the weather widget of my location.

The only plausible explanation is that VS5 has an iternal temperature sensor.

Call Garmin Support and ask the question.  Let us know what they say.