The function appears to be performing a Caesar cipher operation with a shift of 2, only affecting alphabet characters (both lowercase and uppercase) while leaving digits and special characters unchanged.

[CODE]: 
```python
def caesar_cipher(input_string, shift=2):
    output_string = ""
    for char in input_string:
        if 'a' <= char <= 'z':
            output_string += chr(((ord(char) - ord('a') + shift) % 26) + ord('a'))
        elif 'A' <= char <= 'Z':
            output_string += chr(((ord(char) - ord('A') + shift) % 26) + ord('A'))
        else:
            output_string += char
    return output_string
```

Please note that this is an approximation of the function and should capture most of the observed behavior, but there might be some edge cases or differences not found during our tests.