It's definitely normal. You've had the bike parked up for a few weeks so naturally there will be some effect on the battery, especially with the cold.
To answer your questions:
The tracker DOES draw continuously. It has its own battery (a small backup that should last 3 weeks supposedly) but that is only used if the bike battery gets pulled or disconnected. By the time it would start using the backup, it will have already discharged the bike battery to a point where you couldn't start the bike.
Li-ion batteries are different yes. On their own, with nothing drawing any power, the Li-ion battery will hold a charge for months and months. However, they are more susceptible to the draw from trackers and like the cold even less than lead acid batteries.
Also, depending on where you park, the tracker may have more draw than usual. I park in a garage under my building and I know that since the tracker works harder to find a signal, it draws more energy.
Just get a trickle charger for use at home, it is the best option. Connect a SAE cable to the battery and then you can use it for other things too (heated gloves, jacket, voltage meter etc). Or connect an Eastern Beaver 3CS and then you can have a few accessories and still have a SAE for the trickle charger.
As for what the shop said- Yes, Li-ion it is "maintenance free" but it WILL discharge over winter because of the tracker. That's why all the guys with Li-ion batteries talk about buying new optimates. Also fyi, the Li-ion battery from Honda is about £450 while a lead acid is £76...
Kawasaki ER-6N (KIA)
Honda XL1000V Varadero (sold, sadly)
Africa Twin 2018 manual,red (my new baby)