It does not look anything like a 'loose panel' or a hatch that has been left open.
You and I know it's not a loose panel, at least not from such a photo taken from under the aircraft, but to say it doesn't look anything like a loose panel or hatch is stretching the truth a bit.
Some maintenance hatches are not much bigger, but
most of them are on the fuselage centerline. e.g. rear stabiliser access panel on a 747. The A330 fuelling panel is off-centre, but well underneath the aircraft. The forward equipment centre hatch on an A380 is also off centre but opens inwards, I recall.
The red outline may be a give-away (to show a hazard area), but an orange/red colour scheme might hide that somewhat.
Tower: "Airline XXX, it looks like one of your baggage doors is open."
Captain (after quickly scanning the FE panel): "Ah, thanks tower, but you must be looking at our APU door."
I remember hearing a story of a 747 Classic flight attendant calling the cockpit just before takeoff about something hanging from the wingtip. The flight crew incorrectly misinterpreted his description of the item as a static wick/discharger. It turned out to be a HF wingtip antenna (probably now resting at the bottom of the ocean). How long were those things? 2 meters? Made of aluminium, but it still would have put a decent sized hole in someone's roof.