[GT-Rpilot]

Concur. Previous owner likely half-assed a MP DIY replacement back to stock to sell the part off, and left you with the results, either by not properly tightening the bolts (they came loose) or by simply not putting them back in at all. Did you buy directly from owner or through a dealer? If the former, I would contact this person with questions. If the latter, then it is what it is - get some new bolts put in. Regarding the undercarriage tray fasteners, they have a known history of corrosion - I don't have the thread handy, but there are some companies that sell a titanium replacement kit that includes all new hardware for this very case. You can do a search for that.

 

[GT-Rpilot]

More than likely it was tuned, then re-flashed with a "stock" tune when the parts were removed. A tune via Cobb AccessPort or EcuTek is required when adding 3" intakes which is one of the very first mods owners do - typical entry-level mods for "best bang for the buck" power increase is intakes, Midpipe and tune. After that, it's replacing the downpipes and injectors for the basic "FBO" or full bolt-ons upgrade. Adding a catback exhaust system doesn't really yield much power, but a lot of people do it for better sound or to lose some weight (full titanium). When an owner wants to go back to stock, they get a base tune that emulates the stock configuration, so the OEM parts will work again - generally easier to trade a GT-R in if it's not modded, and a lot of owners likely don't even disclose to the dealer that it was ever modded. Even stock, these cars produce soot, but it's much worse when modded.