When you send bitcoins to an address the remained amount is moved to a new address, which is not listed anywhere but it's includes in your total sum. I summed up the amount in all the addresses I had the "leftovers" moved to and they equal to this "mysterious" amount I was wondering where it came from.
I don't know if there's a way to see those used addresses somehow in the Trezor Suite other than exporting all the transactions in the file.
Cheers
Side note: Using the XPUB as a watch only, does not include those hidden addresses, just those generated by the user to receive funds. You can still add those addresses manually if you take them from the list with all transactions.
Curious why you are not using Electrum as NeuroticFish suggested above. I do that also. Many elect to use only the Trezor site but that is a glaring problem as you found out. Also, as a side note if privacy is important to you I would argue that Electrum servers are better than using only one server from the "mothership". Many users have no concerns/needs regarding being private and only want to know they are secure. In that case either option is very secure in tandem with a hardware wallet!
It's just because I don't use my Trezor so often to connect it here and there. That's why I have the Sentinel , just to monitor the amount in the wallet and watch some other addresses. So basically my Trezor is connected as fewer times as possible to any Device with Internet. I update the firmware when is needed of course, and I'm thinking of buying one more Trezor wallet to have a backup device in case something happens with the one I have now. The way to export the transaction history in a spreadsheet is working good enough.
Thanks guys for the help, at least I was happy surprised to see that I have more coins than I expected :-D