i'm in the same position as you, my thermostat was replaced around this same time last year, after picking up the car and driving on the way home i lost all power. VW sent a technician out to me as i was stuck at the side of the road and said they had not put the hose on properly. After few 100 mile's i noticed the smell again, i thought its the residue, another few 100, i thought nah there is something not right here. So back to VW, they say its residue, got the car back and smell seemed to of gone, month later, it's back again (now we're in april lockdown), i get car in air, noticed residue underneath, clean it all off, test car out still the smell. get it into another dealer who said cant find no issues etc etc. car goes in for service, on the health check video, he says signs of residue of a leak (now i've cleaned this prior), so i said look, even your tech has said there is issue. same "no issues to find, we've done pressure tests, run the car for hours, driven it for hour or so, no faults" in the end it was left by the dealer owner that if the smell happens when im out on the road, then to call up, ask for him direct and drive right to them so they can smell it and go from there. in total it's been in about 3/4 times after the intital repair with regarding the smell issue. VW customer services were aware of all this, but cant say on "common problem as not flagged by vw as one"
i've been on furlough since november and not driven the car, basically sat parked up. I've got to the point of, i'm going to end up sending to an independent to get looked at and if so sort it (quotes from 500/600 onwards - to do both water pump & thermostat - revision parts). not sure if i can take up anything after cost wise being helped, but i cant keep dropping my car off at vw in morning, for them to not start looking at it till end of the day or the day after, then same answer back, least independent will work on it right away when booked in
i have though had no coolant warning light, nor seen any drop from the coolant tank. just this stupid horrible smell. i had read few others say they've had this smell and say "it's normal" which i don't think it's right.
hope you get it sorted