The IRS does not know (unless you get unlucky enough to be audited) but the legal thing to do is
(a) to put it back or
(b) report it as taxable income or
(c) use the money for a future medical expense (still in 2016) and then not request reimbursement for that expense. As long as your total qualified expenses for the year are equal or more than your qualified withdrawals plus the amount of the refund, then the refund doesn't matter.
Note that if you want to put the money back in the account, you will need to contact the HSA administrator and tell them in advance that this is a return of a previous withdrawal. There may be a special form to send with the check. Otherwise, sending them a check will look like a new tax-deductible contribution.
If you decide to keep the money and report it as taxable income, it is in the very bottom section of miscellaneous income under "reimbursement (or recovery) of a previous deduction".
