Specifically: cell phone not connected to internet, not used to talk, just powered on. What if it's powered off?
When a cell phone is on, its radio talks to base stations, so even at a simple level, the carrier knows roughly where the phone is.
Using triangulation and signal strength to nearby stations you can get a very accurate location fix.
If your phone is turned off then it won't be found (unless it is a target of high level espionage, in which case it might be communicating...
Even further to what Rory and Andrew said, a cellphone could indeed be detected even if completely powered-off, with battery removed.
Every cellphone contains multiple antennas, which are tuned, resonant circuits. You could (in theory, with the proper equipment) detect the presence of a cellphone in a given direction with radar techniques, detecting a peak in reflection at a particular frequency (maybe slightly off the nominal frequency of the antenna, or where it's supposed to be more untuned/reflective). And with triangulation, you could pinpoint the precise location of the powered-off cellphone (or of multiple cellphones)
Science fiction? Well, just consider that it's similar to the cheaper technique used in RF security tags (where absorption is measured instead of reflection)... If you need an analogy, the difference between RF security tags and what I'm talking about here is the same difference that there's between radiography and backscatter x-ray imaging.
Of course in this way, if the cellphone is totally powered-off, you will only be able to know that there's a cellphone there. You will not be able to identify the cellphone. But again this is supposing there's no backdoor in the cellphone RF chips enabling them to work like an "high-distance RFID tag"... Beaming high power RF to the device could give it enough juice to let it reply with an IMEI...
Is this even more science fiction? Well, something similar it's been done in 1946. Are you really sure a proper passive, high-distance identification is not feasible with today's technology?...
As well as Rory's answer, many modern phones now have GPS on board and many use Assisted GPS which broadcasts the position.
Also remember that a "switched off" phone may not be entirely off (more of a standby) so may still be maintaining some form of contact with the network. Only if you remove the battery can you be sure it is powered-off
If powered off you could NOT detect the IMEI. End of story.
With a very focused transmission over a short distance you may be able to detect absorption reflections, but any other wiring or electronic devices would certainly produce enough false positives for this to be unreliable even to 007 over a usable distance, you certainly couldnt detect a specific cell phone.
If were playing the 007 game, you could however bug a cell-phone, a SIM card or micro sd card could be modified to do this in theory, but you would probably need the resources of a fab to achieve in practice.