What this can and cannot tell you

For a Skep it reads lastGleanBlock(), gleanCount() and ripe() — the vault publishes them, so the answer is exact.

For anything else there is no such view, which is the point of this page. What it can do is compare the asset balance the vault holds against the number its own totalAssets() reports, and the difference is a lower bound on income sitting uncollected. It is labelled a lower bound rather than a measurement, because some of that gap may be a donation or accounting this gauge cannot see.

A vault that does not answer asset(), totalAssets() and totalSupply() is reported unreadable, not empty. Those are different answers and a decoder turns the second into the first: an empty return decodes as zero, and zero is a perfectly good number here.