HI SAM! I am considering two angles. #1, a spells that can be cast once per LR like you'd find in a normal racial spell casting ability. OR #2, additional spells to add to your spell list from other class' that relate to stealth. Some cool stealth spells are not available for Artificers. Also, we could think outside the box of existing formats and create a unique ability if these done jive.
OPTION 1: RACTIAL SPELL CASTING FORMAT
Once you reach 3rd level, you can cast Invisibility once as a 2nd-level spell. Once you reach 5th level, you can also cast the Pass Without Trace spell once. When cast, neither of these spells requires concentration. You must finish a long rest to cast these spells again with this trait.
NOTE: Usually this racial ability also adds a cantrip. I didn't see an existing cantrip that screamed Thomas and related to being unknown, unwanted, unseen... Any ideas? My two ways to buff was they both ignore concentration, and typically an artificer wouldn't be able to cast invisibility until 5th level, so having it at 3rd would be pretty good.
OPTION 2: EXPANDED SPELL LIST (could be modified as we play)
Starting at third level, you have certain spells prepared. These spells count as artificer spells for you, but they don’t count against the number of artificer spells you prepare.
3rd Level: Fade from Thought
(1st level spells) consider... Fog Cloud
5th Level: Pass Without Trace
(2nd level spells) consider... Silence, Invisibility (already artificer spell so maybe not)
9th Level: Blink
(3rd level spells) consider.. Nondetection
13th Level: Greater Invisibility
(4th level spells)
17th Level: Modify Memory
(5th level spells)
NOTE: as you can see Fade from Thought is a homebrew spell. There wasn't any other hidden spell that existed for level 1 that I saw. It's something that hopped to mind when thinking about Thomas. Also I did 1 additional spell per level, it felt more balanced than 2. And you can see I added some that I was considering.
Credits: Created by Max for Sam's character Thomas in the Fate's Embrace Campaign.