I feel like I figured this out when I last saw it about 2 years ago, but
can't recall the answer:

When I open my workbook today, I get an update links query. When I go to
edit links it says the problem is atpvbaen.xla. I seem to recall this is
some sort of add in, but it shouldn't appear to be a link, I don't think.

What is causing this and how do I remove it?

It seems like it's caused by a new worksheet that has eomonth functions
which are an add-in. However, I already used this same function in other
old sheets and didn't have the problem (at least not in a couple of years of
my memory)!!

Thanks!
Dean