I would suggest you compile a list of the missing assets and the ex-staff members they were allocated to, along with the value of the items. Present that list to management with three options: write off the cost, try and pursue the people to retrieve the assets, or call in the police. I suspect the police will not be interested until you have explored all other avenues to retrieve the items. If this is a long-standing issue and the chances of recovery are slim, I suggest writing it off. You may be able to get a tax advantage through a write-off. I do not know Indian Tax Law.
Starting Now: Asset Register Implementation
Put in place an asset register of all the company's equipment, particularly anything that is held by a staff member, such as a computer, mobile phone, etc. Record the name of the person, serial number of the item, value, complete description, etc. Every item should be tagged with an asset number. There are many companies around who specialize in providing suitable asset number tags or stickers. Ideally, they need to be hard to remove. Items can also be etched with an engraving pen.
Contractual Agreements with Staff
I would also suggest drawing up a contract/agreement with the staff member, leaving no doubt whatsoever that the company owns the item(s) and they MUST be returned upon cessation of employment.
Regular Asset Register Checks
Most importantly, do a regular asset register check. You need to know at all times where the items are. If a person leaves the company, part of your separation process is to ensure you get back anything they have that belongs to the company. If necessary, make payment of the final salary dependent on returning the items.
If a person absconds, then that is theft, and you should call the police immediately.
Hope that helps.