I want to detect where my stolen laptop is. If I synchronize the laptop with mobile phone using bluetooth so that when I login to my system it sends an alert message to my phone. f Now the issue is if the laptop is stolen and if the culprit already knows the login credentials of the laptop and the bluetooth range is not enough to work, how will i be able to get the alert message to the phone?
Well Bluetooth implies that you are close enough (within no more than around 30m line-of-sight) to your stolen laptop that you can make a connection. That doesn't seem very robust as an anti-theft process.
The way it is normally done is that the laptop runs code that reports it's IP address regularly to a central location.
If you want to create your own code, it isn't difficult to do with Node.JS or Python and an Internet facing MQTT broker. MQTT is a pubsub service so you can have a simple client on your phone or indeed a monitor service on a server somewhere that will check for IP address changes and alert you. There are many other ways of course to do the same thing.
An IP address is not that reliable a measure of location of course so there are still significant limitations in this approach.
You could further improve things by finding things on you main networks (the ones where your PC will normally be found) that the service can ping to check that it is where it thinks it should be.
UPDATE: If the PC is not connected to a network, certainly Bluetooth would be better than nothing. But it is very unreliable. Having some kind of Bluetooth device permanently on in fixed locations might help (though please update your Q with more details so we can better help), perhaps a Raspberry Pi with a Bluetooth dongle. You certainly can't do that with a mobile phone because the phone will likely turn off BT when on standby (e.g. most of the time) to save battery.
The other issue with BT is that it doesn't cope well with multiple connections. So a fixed BT monitor would work OK maybe for a single PC but not for many.
What you should do is set up a local Wi-Fi network with the PC's connected to it and have a central monitor watching for PC's dropping off. But how do you differentiate between a theft and someone leaving the office?
I suspect you really need something to monitor for when a PC appears on an unknown network and that was the focus of my original answer.