was that a stock intake manifold? if so thats not a factory hole...what kind of car is it? you can check the trunk for where the bottle would have been mounted but i have seen people hide two 2lb bottles in the door too so if they dont want you to find it you wont. if you have the proper tool you can do a compression check on the spot. nitrous is fairly easy to remove and install a nitrous system, look for holes in the firewall from mounting solenoids, generally people will use small self tapping screws, might also be on one of the fenders.