When using keyboxd, as well a in recent VSD versions, there is no "No secret key" error shown for S/MIME encrypted data if the secret key is missing.
Instead the corresponding error for S/MIME is "Not found":
C:\Users\g10code.WIN-TEST3\Documents>gpgsm -d test.txt.p7m gpgsm: failed to find the certificate: Nicht gefunden gpgsm: message decryption failed: Nicht gefunden <Keybox> [GNUPG:] FAILURE gpgsm-exit 50331649
"Not found" here is no user friendly error message, it begs the question "what was not found?".
As we do use the "No secret key" error for OpenPGP in the corresponding case and we also did give that error for missing S/MIME keys in old Kleopatra versions without keyboxd I believe we should show the same error for both protocols.
In old VSD versions (checked for 3.1.26) it looks like this:
with the gpg part:
gpgsm: failed to find the certificate: Unbekannter Systemfehler gpgsm: message decryption failed: Kein geheimer Schlüssel <GPGSM>
The "unknown system error" was fixed meanwhile, but the "No secret key" error message got lost since then, too.
In the current vsd version 3.3.4 it looks like this:
with the gpg part:
gpgsm: failed to find the certificate: Nicht gefunden gpgsm: message decryption failed: Nicht gefunden <GPGSM>