If you are looking for the physical piece of equipment, then there is no option but to send out a search party to look for these machines.
I conclude you do not asset tag your machines or record Machine ID to username, so cannot perform an audit of the hardware that has been used in recent days, and figure out which asset numbers have not been used, then track back to the actual user??
There are software tools such as SMSWakeUp which can power up a PC if it is LAN connected and plugged into the main power supply, but if the equipment has been stolen, then you won't be able to access it at all.
Sounds like your company has lost control of its hardware inventory.