Agnosticism isn’t “I don’t know if gods exist”. It’s “It’s impossible to know if gods exist.” It’s not a statement of belief about the existsnce of gods so much as a statement about the availability of evidence of existence of gods.
Being a different sort of belief, agnosticism does not contradict either atheism or theism. If you say “I can’t be sure, but I don’t think there are any gods”, then you’re both agnostic and atheistic. If you say “I’m pretty sure there are no gods”, then you’re simply atheistic; if you say “You can’t know whether there are god, so I’m reserving my opinion”, then you’re simply agnostic. An agnostic theist would say “I believe in a god or gods, but have no reason to; no such information is available.”
Now, if you’re theistic because you believe you have seen evidence that a god exists, then you’re not an agnostic, because you believe that you’ve found evidence which imparts knowledge about the existence of a god. Agnostic have found no compelling evidence for a god, but cannot be certain of godly nonexistence and are unable to ignore the massive amount of unsearchable areas where a god could theoretically be hiding enough to assert that a god doesn’t actually exist, hidden away someplace.
When dealing with witches and peanut butter monsters, you can accumulate enough evidence to become reasonably certain of the negative. (You don’t need absolute certainty since such certainty is generally unavailable in life, just reasonable certainty.) You can watch the vents or the peanut butter, personally or via cameras, checking for dust or whatever, and determine to your satisfaction that there are no monsters crawling around there. For witches, you can study enough cases and find likely and plausible other explanations, or fail to find supporting proof of witchery, until you feel that you’ve checked enough of the supposed cases to be fairly sure that there’s probably not some valid case you’ve missed.
Gods, though, are another matter. Putting aside those that supposedly live on top of mountains or in the core of the earth or other such examinable places, it’s quite common for gods to be described as living ‘elsewhere’, outside of the observable universe. Now, it’s quite hard to observe the areas outside the observable universe, so we can’t even start to check around the places where gods might be hiding to see if there are any there. Similarly, the supposed actions of gods on earth are commonly described as leaving no other detectable traces, and often as looking a lot like other, ordinary events, and/or turning up in the few areas we still have a spotty understand of, like the human emotions or mind. Again, the slippery evasiveness with which gods are attributed typically make it impossible to conclusively eliminate the possibility that there’s some god skulking around doing things ‘undercover’. Agnosticism is what happens when you determine that the limits on your knowledge exemplified by these sorts of things makes it impossible to reach a firm conclusion.
Personally, I’m atheistic about the Christian god and various other specific gods (for the usual many and obvious reasons), but agnostic about ‘other’ more subtle or noninterfering gods which may or may not exist. In other words, I get tied up in the unavailability of information about such gods and have thus decided that one can never be certain about their existence or nonexistence.