BQN, 30 28 bytesSBCS
{∾•Fmt¨×⊸/+˝(⥊𝕩-⌜"aA")=⌜↕26}
↕26 Range from 0 to 25.
⥊𝕩-⌜"aA" Differences between each of the characters in the input and a or A.
=⌜ Equality table between those two vectors.
+˝ Sum the columns.
×⊸/ Keep the values with sign 1 (or: remove the zeros)
∾•Fmt¨ Convert each value to a string and join.
A slightly different approach using Bins Down at 29 bytes:
{∾•Fmt¨×⊸/»1↓/⁼27↕⊸⍋⥊𝕩-⌜"aA"}