Heres more photos i also did try to put my finger underneath abit but can only do so much with the matt. It does look abit different in differenet angles as seen in the photos.
Well, the good news is that there’s no chip in your tank’s glass!
My best guess here would be spilled water leaking down the viewing pane, hitting that protruding mat edge, and creeping under the tank, between tank and mat from there!
Can you stick the edge of a tissue in the crack between tank and bent-down mat in that corner to see if it comes out wet? If so, that will confirm this is indeed water…
To determine confidently if water is from a spill, or a leak; do your best to dry things off, then check again for any new moisture before you put hands in the tank again!