OK

When SQL compares column values with a variable value where the values are diferent data types.

There are 2 posibel methods:

1 Coerce each column value as it is read row by row from the table and compare to the variable

2 Coerce the variable value, cahe the coerced value. Compare tow by row the column value with the cached coerced value

The most efficient method would appear to be 2 as the coercion is only done once.

ALSO

My test database has always had a column with the value 'AB' - and yet the SP has never failed before.

There is clearly something odd going on here. MY suspicion is that it to do with how SQL optimises qurey plans