Holy cow this thread.
Ok, so, it is almost certain that the BLE (Bluetooth Low Energy) modules on the side pillars and rear bumper are used for the BLE PXP (proximity) profile. This is a proximity sensing profile that allows devices to detect the presence of another BLE device. This also gives me hope for a feature I'd love to see- unlock the mobile device to unlock the car. Because the sensing of the presence of a device allows the mobile OS to present notifications or other actions, it could easily request that iOS or Android force the user to unlock the device to allow access to the car. Tesla: A simple checkbox to enable/disable this would be great. Thanks.
As for the number of sensors, this could be for lots of reasons. It could be useful for RSSI (Received Signal Strength Indicator) filtering which is roughly how proximity is calculated, it could be useful to detect where a user is approaching from, it could be useful to verify a sensor isn't being attacked with a directional antenna of some kind, it could be useful for detecting proximity in an area with lots of devices or signal noise, I could think of a thousand reasons. And, since the sensors are so cheap, ($2.15 or less per thousand, with progressive discounts as volume increases), there's basically no reason not to add a bunch of them. For a million cars, and the maximum price, these are still only $2,000 total component cost.
For the people thinking Tesla would use BLE for radar. No. Just no. Stop it. The wavelength for 2.4GHz is 12.5cm. Bosch (and Continental) MRR/LRR systems use 76GHz frequency which is a wavelength of 0.39cm or 3.947mm. The resolution is almost 32x better. Not only that, but the noise on the 2.4GHz band in a car that uses bluetooth, with a phone that scans for WiFi, on roads with cars that have WiFi hotspots in them driving by, it would be a signal filtering nightmare. And the chips involved simply do not have that kind of processing power or features onboard. The datasheets are freely available from Texas Instruments
here. Check out the device and the development kits available. They're super simple devices that do a good job at one thing. Bluetooth LE.
One big positive here, this does mean at some future date Tesla could potentially build a FOB or a very small pocket bluetooth device that could replace a phone or the card key. The batteries in Bluetooth Smart or low powered BLE devices last for years at a time. It's basically how Tile and similar products work. If you want to learn more about PXP profile or other BLE GATT (Generic ATTribute) profiles, see the bluetooth.org GATT page
here. There are tons of them, and lots of them have very cool features that can be implemented at the same time. This is all pretty "boring" standard stuff that has existed for a long time. Tesla, Apple, and Google/Android are simply adopting these existing technologies better and faster than everybody else, and using them in interesting ways. This is much better than a rear-facing radar system, because this vastly improves the user experience of the vehicle!
Edit: I defined abbreviated terms, because not everybody here is a software or hardware developer. Sorry.