I do lots of vlookups, but I have encountered a problem of a different
shape.

The piece of data that I am looking for may occur anywhere within a 12
x 16 range (B3:M18). And all I need is to confirm its presence or lack
thereof.

So, look for the value from A1 and check the range b3:m18, if found
TRUE, else FALSE.

Which function do I need to do this? Is there one?