A proposal for a new logical fallacy, or does it already exist?

I think what you describe is similar to the example in the ad hoc rescue . 
Example
Frieda: I just know that Raymond is just waiting to ask me out.
Edna: He has been seeing Rose for three months now.
Frieda: He is just seeing her to make me jealous.
Edna: They’re engaged.
Frieda: Well, that’s just his way of making sure I know about it.

Yes, it’s called the dog chasing its own tail fallacy and the reason you can’t quite catch  it is because you haven’t figured out that it’s attached to your own arse.

Good luck!

There are about a dozen fallacies that this could fit: circulus in probando, moving the goal post, ad hoc rescue, etc. 

Unfortunately, you haven’t found the answer yet because you don’t even know what the question is.

So, start from the beginning.

What’s the claim? What’s the question? What’s the argument? Where’s the deception? 

Go!

Hi, Epsodom!

      The example you provide, in which Bob assumes that there was a million bucks on the porch for a whole hour, resembles a parable written by Antony Flew in his article called Theology and Falsification.

 
      Flew spins a fable of two explorers hiking through a jungle. They happen across a clearing that is filled with flowers and weeds. One of the explorers supposes that there is a gardener who tends to the clearing. The other explorer thinks there is no gardener. They set up camp all night and wait to see if a gardener comes around; none ever does. The “Believer”, Flew calls him, responds that the gardener is invisible. The two explorers set up electric wires and have their bloodhounds patrol the clearing to try and capture the “invisible gardener” and thus reveal his existence. But no yelps of pain ever occur to suggest that an invisible gardener tried to bypass the electric fence, and the wires never shake to suggest that an invisible person attempted to climb over them. The Believer says that the gardener is immune to electric shocks and is intangible, which is why no yelps of pain were ever heard and the wires never shook. The dogs never bark (but they would have barked if they smelled or heard a stranger such as the gardener.) The Believer responds that this is because the gardener is extremely quiet and has no odor. Initially, the Believer simply believed that there is a gardener. By this point, however, he maintains that there is an invisible, intangible, extremely quiet gardener who has no smell and is immune to electric shocks and takes care of the clearing.

      The skeptic asks the believer what the difference is between a gardener of that description and an imaginary or nonexistent gardener. When qualified in so many ways, it seems that the utterance “there is a gardener” does not really amount to meaning that there is a gardener. When so heavily qualified, the utterance can’t even capture the statement that there is a gardener rather than the statement that there is not a gardener. "There is a gardener" has become a vacuous utterance.

      Flew is using the parable to support the falsification thesis. This thesis says that an utterance about the world is meaningless if there is no sensory experience that could count against it. This is basically because an assertion about the world is equivalent to the denial of its negation. There is a gardener is equivalent to it is false that there is not a gardener . Due to the equivalence of these two statements, the meaning of the assertion "there is a gardener" is tied to what it would take for there to not be a gardener. Yet the believer seems to allow nothing to be incompatible with the claim that there is a gardener. For even the lack of visual, tangible, auditory, olfactory sensation of a gardener is still compatible with the existence of the gardener, so the believer maintains. Since the positive utterance about the world (“there is a gardener”) seems to have no empirical circumstance constituting it’s negation, the positive utterance is meaningless. The issue here is not that the Believer is committing a fallacy, but that his utterance is vacuous (according to the falsification principle.)

Perhaps Flew's thesis and argument are what you are reiterating?

Thank you, Epsodom

From, Kaiden