Agnostics are Correct

Agnostic people often conflate "negative observations" with "absence of evidence".

To take the Santa example - if you simply declared "I have no evidence that Santa exists, therefore he does not exist", then you would be arguing from an absence of evidence.

However, if you said "Santa is said to travel in a flying sleigh, and no radar returns consistent with such a vehicle have ever been observed" then this is not an absence of evidence. This is a hypothesis (namely, that Santa flies around the world in his sleigh) from which we can make a prediction (that the sleigh would be visible on radar) and then we make an observation that the predicted scenario does not arise.

This is a negative observation - it's not an absence of evidence, it's evidence that our hypothesis may be incorrect.

Of course, you could argue that the sleigh is magically hidden from radar by the pixie dust mixed into its paintwork, but at some point Occam's Razor kicks in and reminds you that the simplest explanation for a negative observation is that the thing you were expecting to see simply doesn't exist.