There are a few things in your first post initiating this discussion that contain inappropriate words, giving a wrong meaning to a reader.
1. You should have adopted some systems by which your office will have some documentation for whatever office materials one is taking out. I'm aware that even small items such as a pen drive or a floppy disk onwards are documented through a "Pass out," duly authorized by a designated officer and/or entered in a register. Security passes, and the receiver acknowledges having taken them out of the office. If so, these records should be handy to establish that a laptop, data card, etc., were issued to him. If nothing is available, it would be difficult to prove that these company properties were actually issued to him for company purposes and return to the company when done or on exit, whichever is earlier. He might as well argue as if they are issued permanently for his use and not meant to be returned. Where are the terms and conditions governing such transactions?
2. It's almost nearing one month since he resigned, and the one month's notice period is coming to a close. By 9th April, you have to decide about his resignation/relief. If you are not relieving him of his resignation, it would be construed that he is still on the roll and 'dies non' would arise. It also would be deemed that his resignation is accepted since you have not sent non-acceptance or acceptance of his resignation so far. No inquiry is initiated. You'll have no right to detain where no inquiry is ordered.
3. As some pointed out, unless you have a very strong case of theft of company property, it is very difficult to enforce an FIR, as the fact is that 'laptop, data card, mobile,' etc., were actually provided by your company to perform certain duties correctly. Not returning them is a different issue which can be easily linked to non-payment of salary and non-acceptance of the resignation letter, yet to be relieved. He may as well argue he continues to be your employee as his resignation has not yet been accepted.
4. Similarly, the delay in payment of salary itself could be adduced as a reason for resignation.
So filing an FIR could be a last resort, and a domestic inquiry cannot go side by side parallelly on the same charges.
If no other serious charges are with you against him, better settle it amicably on a 'give and take' basis.
Regards,
Kumar S.