You can go read the book and do some math yourself.
Sure we can do math ...
So Astarion starts at Dex 17 ... on level 4 its 19 ... using Ethel hair 20 ... that means +3-5
Since he is profficient in Stealth its +2 ... +3 on level 5 ... but he have Expertise there so actualy +4 resp. +6 ...
Drow Leather armor that is simply looted from container, there is no hard fight (Bulette can easily be avoided) or long quest ... you simply come and get it ... gives another +2
Same story goes on that Ring you find on rise road ... another +2
Also Astarion is a Vampire and if he drinks some blood he get +1 til Long rest ...
And just to have it covered all if you kill Duergars, Sovereign will give you Bliss Spores that gives you 1d6 on ability checks ...
There is ofc Guidance that gives you another 1d4 ...
And blessing of trickster wich is cantrip and gives you advantage for Stealth rolls.
So ...
Astarion on level 1-3 if i count corectly have +12 on Stealth ...
On level 4 he have +13/+14 depensing on of he allready used that hair ...
And on level 5 +15/+16 ...
And that still dont include his rolls NOR Bliss Spores, guidance, or blessing of trickster ... wich are all free.
But if we would count them ... we can reach ridiculous +17-26 AND ADVANTAGE ... and that is still just bonus to our roll!
Meaning ... unless i count incorectly ... since we are talking about rolling 1 ...
Our guard would need AT THE VERY LEAST pasive peeception 19+ to discover Astarion.
AND
Even that is only if Astarion would be incredibly unlucky to roll 1 for his skill check, bless and spores SIMULTANEOUSLY ... and there is still advantage from blessing of trickster ... i refuse to calculate chances for that so i just say they are incredibly low.
Shame you refuse to tell how exactly are we suposed to "easily fail" this monstrosity.
