Passaggi, fach and volume

I think the easiest way to find your segundo passagio is to without warming up, or only warming up very lightly, attempt to slide has high as you can without a shift in vocal tract posture. Start at your speaking tone or maybe even a little bit lower. Primo passagio on the other hand do the same but slide down, you'll reach a point where if you're really paying attention that you will have to shift the track to go lower.

My hypothesis as to why it "changes" as you've warmed up throughout the day is because you've already begun to bridge it or have fully bridged it and thus your voice has already coordinated the transition, thus masking it and making it harder to detect.

I have heard the passaggio is a biological feature of the human. That way it should not depend on external attributes such as the time of day, because you are born with your body and passaggio.

I have also heard these, which seem to hold true very well: Basses/altos have their passaggio start at the note A below middle C, and end at the E above middle C. Baritones/mezzos have their passaggio start at the middle C, and end at the G above middle C. Tenors/sopranos have their passaggio start at the E above middle C, and end at the B above middle C.

So your voice starts to "change direction" at the note where your passaggio starts. The voice must always be kept in the mask / in the front of the face, but the space grows inside your mouth and this is where the voice "direction" goes as you sing higher notes. For the highest notes, your voice must be kept in the front, but you should feel the space in the backside of your mouth in the same place as if you were vomiting or sneezing or yawning.

It is possible to determine the passaggio with simple voice exercises, if your voice is well enough in the mask / in the front.

Interesting point(s)... To answer, at very least, your first question: "I was wondering how the locations of the passagi are to be located"

The easiest way to do this is to simply sing along with the notes of a keyboard... This was how I was shown the same thing at an ACDA conference when I was singing in choirs as a kid.

Basically find the lowest note that you can sing without straining or changing your voice in any way (singing from your diaphragm,) then begin to ascend the keyboard chromatically; following the fingering with your voice. You will naturally find the Passaggio. Again, change nothing in your voice or body-posturing in order to compensate for the inevitable change in vocal register that will subsequently happen (it's almost a subconscious reaction to try to prevent the Passaggio.)

As long as you don't try to prevent it artificially/subconsciously, the Passaggi should stand out like a sore-thumb...

Online searches don't reveal much, or perhaps I'm just doing it wrong, but I did find this post as a justification:

Your body and mind will tell you when you've reached the first pitch in your primo passagio. It is the point where an untrained singer will either crack into falsetto or start yelling to go higher. Neither is correct of course. For the majority of male voices it is around D#3 but basses break around C3 (middle C)/C#3. I'm a fairly light tenor so my break is usually on the E. Voice type is based mainly on timbre and in classical music tessitura (the note range where you are strongest and most comfortable). So if you continue singing classical your fach is quite important as it will determine the classical repertoire you'll sing and the parts written for your fach if you will be singing Opera. But look at Placido Domingo. He was what his teacher called a lazy Baritone. He then "switched" if you will to Tenor. So anything is possible especially considering your experience level and age. Don't sweat it yet. Just study, learn, practice, explore your instrument and have fun with it.

EDIT

Reflecting on this after the fact... try the keyboard exercises during different times of the day... and, at different volumes... Ignoring what I said before - change your body posture, try to prevent it artificially, try to force it... change your body posture again and then don't force it; do all of the unconventional things. You might change the game in doing so. (Just don't hurt yourself, physically ;)