Your not robbing anyone. You paid for insurance.
Wrong! What you are advocating is called Insurance Fraud.
The original claim was legitimate (not fraud). But as soon as the lost item was found, keep the replacement iPad became insurance fraud. The OP has a duty to inform UPS & Apple and return the replacement.
Concealing the fact that he no longer is entitled to the insurance settlement is Insurance Fraud.
Not just my opinion. Laws and definitions vary by state, but here is what the California Penal code says about it:
550(b) It is unlawful to do... any of the following: ... (3) Conceal, or knowingly fail to disclose the occurrence of, an event that affects any person's initial or continued right or entitlement to any insurance benefit or payment, or the amount of any benefit or payment to which the person is entitled.
http://www.leginfo.ca.gov/cgi-bin/displaycode?section=pen&group=00001-01000&file=548-551
So the OP is now "concealing an event" (lost iPad showing up) which affects his continued entitlement to the insurance benefit (replacement iPad).
Punishment?
c) (1) Every person who violates paragraph (1), (2), (3), (4), or (5) of subdivision (a) is guilty of a felony punishable by imprisonment pursuant to subdivision (h) of Section 1170 for two, three, or five years, and by a fine not exceeding fifty thousand dollars ($50,000), or double the amount of the fraud, whichever is greater.
So is it worth facing a criminal record, a $50k fine, and imprisonment? These are some pretty serious penalties. Ignorance of the law is never a valid defense.
What should I do? If I keep this, would they track me down?
Since if they "track you down", you could be subject to some severe penalties, I would recommend you do the right, moral thing, and return the goods which you are not entitled to.