#><> (Fish) 70 bytes

 #.0+4*a5;!?l|!?f4*b+l1--naolc3*1+0.01?!|~ed+0.0+2e-{:;!?+1:i-1:r}+2:"

Probably the longest ><> 1 liner I've ever made.

It will print the output for each character found on a separate line (0 indexed).

A non found character will always print the length of the code + 1 (I could change this if deemed not okay in it's current state) so in this case 71 will always be the "Not found" characters.

I'll run up an explanation once I get the time.

Some test cases;

##K = 1\n1\n71

#"# = 1\n69\n1

Try it online

><> language

><> (Fish) 70 bytes

 #.0+4*a5;!?l|!?f4*b+l1--naolc3*1+0.01?!|~ed+0.0+2e-{:;!?+1:i-1:r}+2:"

Probably the longest ><> 1 liner I've ever made.

It will print the output for each character found on a separate line (0 indexed).

A non found character will always print the length of the code + 1 (I could change this if deemed not okay in it's current state) so in this case 71 will always be the "Not found" characters.

I'll run up an explanation once I get the time.

Some test cases;

##K = 1\n1\n71

#"# = 1\n69\n1

Try it online

><> language