I need to indicate a missing # in a series

[A Sunfire]

I have an inventory list with inventory tag #'s associated with it. I would
like Excel to indicate when a # in the sequence is missing.

 

[Fred Smith]

It depends on what you define as "missing", and how you want this indicated.
What I do is use a conditional format like:
=a2<>a1+1
and choose a highlight color.

Regards,
Fred