The use of UAVs is increasing with plans to eventually integrate them with airways traffic.
At present most of the those logged on Mode S are military so it is worth keeping an eye on Mode S data for any sign of high flying traffic with unusual ICAO/Hex
During the Lybian conflict Predators were often heard on radio , sometimes using the callsign "English" , but were never noted as using Mode S.
However over the last three years or so there have been occassional loggings of Global Hawk
which is known to use the Mode S callsign UAVGH000 and dummy hex 000001
It has been noted across the USA , UK and the Med. cruising at 52,000 feet
In the UK a trial of Watchkeeper was observed earlier this year
it was using false French hex 39C343 generating a spurious reg F-HQ D and also hex D0C343 ...the callsign was ABCDEFGH
It was noted in company with an RAF Hawk at one point.
This year NASA has been testing Ikhana .
Reports suggest that Ikhana has been using full mode S , however so far neither "Ikhana" ABF783 or N870NA have appeared in any logs , despite the NASA testing grounds being covered by Mode S users.
Of course it may be that NASA are using false identification for the trials , but discussion with NASA officials suggests otherwise.
NASA’s Ikhana MQ-9 drone flies with ADS-B equipment for the first time « The Aviationist
"After checking with the project office, I think we have an answer to your
questions about tracking the recent ADS-B system's flight tests on our
NASA's "Ikhana," a General Atomics Aeronautical Systems Predator B
modified for civilian research use, carries the U.s. Federal Aviation
Administration registration number of N870NA, which should be consistent
with ICAO records.
What may have confused you during the recent ADS-B flight tests on Ikhana
was that the ADS-B transponder was squawking a different call sign of
"NASA870 " including a space after the zero.
The flight tests were of the full-up ADS-B system, not just Mode S."
But , a further note was later received
" A clarification to the last line of my prior e-mail. I've been advised
that the ADS-B system we flight tested on the Ikhana UAS was operating
only in "out" or transmit mode, not full-up airborne in-and-out. We were
receiving the "in" information in the Ikhana's ground control station via
the FAA's ADS-B ground system. We intend to test the receiving end of the
system at a later time once that software is developed."
So it looks like Ikhana will in due course be testing Full ADS-B , but so far has been working
within a controlled environment.